[invocation-overhead] Fix; .NET Core support

[jonpryor]

tests/invocation-overhead partially bitrot since it was last
touched in 8602581: it insta-crashes:

% make run
mono64 --debug=casts test-overheads.exe
2021-02-11 17:33:49.479 mono64[90565:6542595] CheckForInstalledJavaRuntimes: Please visit http://www.java.com for information on installing java.
make: *** [run] Error 97

The primary "cause" is d1cce19: it was never a good idea to P/Invoke
into jvm.dll, as:

• The name for jvm.dll differs between platforms! It's jvm
  some places, jre others, and jli in still others!
  If we want (eventual) .NET Framework/.NET Core support, this
  is a non-starter.

• The actual on-disk path is also highly variable.

The solution to this conundrum was to update the java-interop
native library to have a new java_interop_jvm_load() export, and
use that to load the JVM. (Actual determination of which file
to load is left "elsewhere"; once the JVM to load is found, then
we can sanely P/Invoke into java_interop_jvm_load().)

This change never made it to tests/invocation-overhead.

Rework tests/invocation-overhead so that it's less "stand-alone":
it now uses JreRuntime from Java.Runtime.Environment.dll to
create an in-process JVM, using the $JI_JVM_PATH environment variable
to determine which JVM to load. (This is how tests/TestJVM works.)
This change requires "re-structuring" how certain types such as
JniObjectReference work, as it means we must now reference
Java.Interop.dll, which also defines JniObjectReference/etc.
Square this circle by moving various types into the appropriate
sub-namespaces, e.g. Java.Interop.SafeHandles has its own special
JniObjectReference declaration.

"While we're at it", what's .NET Core's JNI invocation performance
look like?

Update invocation-overhead to multitarget net472 and netcoreapp3.1.
Update src/java-interop so that the java-interop native library
can be built as a netcoreapp3.1 library. (This involves removing
all mention of Mono.)

Update Java.Runtime.Environment so that MonoRuntimeValueManager
disposes the GC Bridge, not JreRuntime. This avoids an
EntryPointNotFoundException, as the java-interop native lib
doesn't provide java_interop_gc_bridge_get_current().

Add a new msbuild /t:Run target which runs invocation-overhead
under both the desktop $(Runtime) & .NET Core

make prepare
make all

make -C tests/invocation-overhead run
# Runs tests under mono & .NET Core

.NET Core is faster than Mono for this particular benchmark:

• SafeTiming timing:

  Mono:       9.3850449 sec
  .NET Core:  5.1734443 sec
  

• XAIntPtrTiming timing:

  Mono:       4.4930288 sec
  .NET Core:  3.1048897 sec
  

• JIIntPtrTiming timing:

  Mono:       4.5563368 sec
  .NET Core:  3.4353958 sec
  

• JIPinvokeTiming timing:

  Mono:       3.4710383 sec
  .NET Core:  2.7470934 sec