[XA.Tools.Bytecode] Add Kotlin parsing and honor Kotlin class visibility

[jpobst]

Kotlin Metadata

The bulk of this PR is adding plumbing support for the metadata that Kotlin adds to compiled classes.

This metadata is added to a RuntimeVisibleAnnotation on the class with a key of Lkotlin/Metadata. This Annotation can contain the following key/values:

• k - The kind of metadata annotation (required)
• mv - The version of metadata (required)
• bv - The version of the byte code (required)
• d1 - The encoded metadata (optional)
• d2 - Additional strings (optional)

As of this PR, we only support Kind == Class.

The Class metadata is encoded as an array of strings. First we have to decode the d1 data into bytes using a variety of potential encodings. Once we have bytes, they are formatted as:

• First byte - Length of the string table
  • optional additional bytes to encode length of string table
• Protobuf encoded string data table is next length bytes
• Remaining bytes are Protobuf encoded data

The string data table provides an indexed lookup table used by the Protobuf metadata to retrieve strings from one of:

• Directly encoded in the string table
• The string array contained in the d2 field
• A predefined hardcoded string

The Protobuf encoded data is relatively straightforward. All fields that would be strings are instead indexes into the string data table. It provides metadata about the class itself and every field/method/property/parameter/etc for the class.

Metadata Consumption

With this PR we are starting small to consume this metadata:

• Respect the Kotlin class visibility, so we mark a class as private if it is Kotlin-Private or Kotlin-Internal.
• Kotlin has a lot of interesting non-Class constructs it adds .class files for. Since our code doesn't understand them yet they do not generate usable bindings. Until we have completed our support for these constructs this PR marks them as private so we do not attempt to bind them. If a user really wants to bind these constructs they can mark them as public in metadata when adding the other metadata needed to make them bindable.

Protobuf Generated File

https://github.com/xamarin/java.interop/blob/b6e01bc2bea72f5524f89e0f1f6fede6c13183b2/src/Xamarin.Android.Tools.Bytecode/Kotlin/KotlinProtobufDefinition.cs is generated using this tool from Protobuf.Net:

https://protogen.marcgravell.com/

Paste in this file:
https://github.com/JetBrains/kotlin/blob/master/core/metadata.jvm/src/jvm_metadata.proto

The tool does not support #import so you'll need to replace those lines with the contents of the files:

• https://github.com/JetBrains/kotlin/blob/master/core/metadata/src/metadata.proto
• https://github.com/JetBrains/kotlin/blob/master/core/metadata/src/ext_options.proto

Generate with the default options to create the .cs file.

[jonpryor]

See also: 39a3b87

This PR will also need to update src/Xamarin.Android.Tools.ApiXmlAdjuster/JavaApiXmlLoaderExtensions.cs so that Xamarin.Android.Tools.ApiXmlAdjuster doesn't throw an exception when reading an XML file containing the new annotation XML elements.

...once the new XML elements are emitted...

[jonpryor]

Does this need to be public?

[jonpryor]

This should use .SingleOrDefault().

[jonpryor]

Summary: [Xamarin.Android.Tools.Bytecode] Honor Kotlin class visibility (#499)

Body:

The [Kotlin language][0] stores Kotlin-specific metadata within a
[`RuntimeVisibleAnnotation` bytecode attribute blob][1] with a type
name value of `Lkotlin/Metadata;`.  The annotation can contain the
following keys:

  * `k`: The kind of metadata annotation (required)
  * `mv`: The version of metadata (required)
  * `bv`: The version of the byte code (required)
  * `d1`: The encoded metadata (optional)
  * `d2`: Additional strings (optional)

Add support for a kind value of `Class`.

The `Class` metadata is encoded as an array of strings.

First we have to decode the `d1` data into bytes using a variety of
potential encodings.  Once we have bytes, they are formatted as:

  * First byte - Length of the string table
    * optional additional bytes to encode length of string table
  * Protobuf encoded string data table is next `length` bytes
  * Remaining bytes are Protobuf encoded data

The string data table provides an indexed lookup table used by the
Protobuf metadata to retrieve strings from one of:

  * Directly encoded in the string table
  * The string array contained in the `d2` field
  * A predefined hardcoded string

The Protobuf encoded data is relatively straightforward.  All fields
that would be strings are instead indexes into the string data table.
It provides metadata about the class itself and every
field/method/property/parameter/etc. for the class.

Once the Protobuf data is parsed, we can use this information to
influence the normal XML declarations that `class-parse` produces:

  * Respect the Kotlin class visibility: if the Kotlin metadata
    specifies visibility as `Kotlin-Private` or `Kotlin-Internal`,
    set e.g. `//class/@visibility` to `private`, which prevents the
    type from being emitted into the XML description.

  * Filter out non-bindable constructs: Kotlin has a lot of
    interesting non-Class constructs it uses `.class` files for.
    Since our code doesn't understand them yet they do not generate
    usable bindings for these constructs.  Until we have completed
    our support for these constructs we mark them as `private` so we
    do not attempt to bind them.

~~ Protobuf Generation ~~

`src/Xamarin.Android.Tools.Bytecode/Kotlin/KotlinProtobufDefinition.cs`
is generated by using [`protogen`][2], by providing Kotlin's
[protobuf schema][3] as input.

Note that `protogen` doesn't support `import`, so the input text
provided to `protogen` will need to replace:

	import "core/metadata/src/ext_options.proto";

with the contents of [`core/metadata/src/ext_options.proto`][4], and
replace

	import "core/metadata/src/metadata.proto";

with the contents of [`core/metadata/src/metadata.proto`][5].

The default `protogen` options were used to generate
`KotlinProtobufDefinition.cs`.

[0]: https://kotlinlang.org
[1]: https://docs.oracle.com/javase/specs/jvms/se7/html/jvms-4.html#jvms-4.7.16
[2]: https://protogen.marcgravell.com
[3]: https://github.com/JetBrains/kotlin/blob/master/core/metadata.jvm/src/jvm_metadata.proto
[4]: https://github.com/JetBrains/kotlin/blob/master/core/metadata/src/ext_options.proto
[5]: https://github.com/JetBrains/kotlin/blob/master/core/metadata/src/metadata.proto