Support for authoring type providers which target portable profiles

[dsyme]

In this FSharp.Data PR, we added general support for cross targeting erasing type providers to the CommonProviderImplementation.

This PR brings this support across into the type provider starter pack so it can more easily be used by other type provider projects. The README edits and the FSharp.Data PR explain how the code works. Roughly speaking, we determine the referenced set of assemblies, and if you create your provided objects using

    let ctxt = ProvidedTypesContext.Create(config)
    ...
    let myType = ctxt.ProvidedTypeDefinition(asm, ns, "MyType", typeof<obj>)

then your provided quotations and type definitions will be "remapped" to the approriate set of target referenced DLLs automatically.

I'm also hopeful that we can use this machinery to allow erasing type providers that target .NET Core (when the F# compiler is running on .NET Framework) and vice versa. But we would also need to make the ProvidedTypes code compilable as a .NET Core component, which may not be straight-forward.

This PR also includes some updates from FSharp.Data and some testing for FSharp.TypeProviders.StarterPack.

I also suggest we rename this repo to FSharp.Compiler.ProvidedTypes and make it much more officially the "home" of the ProvidedTypes API, with full documentation, samples and testing in the style of FSharp.Data etc. We could also start providing this component as a DLL assuming there are no loading, deployment or versioning issues with that (quite a large assumption).

p.s. Note that this PR includes the (large) AssemblyReader.fs file, which is independently valuable as an all-F# single-file .NET assembly reader, for cracking the metadata (though not the code) of an IL assembly without using .NET reflection. We may expect Paket file references to appear to this single file (@forki was using one already to the FSharp.Data copy of this file).

[dsyme]

I'd like this code reviewed by people before we accept it - please! :)

@ovatsus After it is accepted I will submit a PR to FSharp.Data to pick up this implementation and check that all tests pass there.

[panesofglass]

Thank you, @dsyme!

[7sharp9]

Erasing is mentioned here but are generatives affected by this PR in anyway? //cc @dsyme