[jnienv-gen] Add possible C#9 function pointer backend

[jonpryor]

Context: 926e4bc
Context: https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-9.0/function-pointers
Context? #666

Commit 926e4bc allowed jnienv-gen to emit multiple different
JNIEnv invocation strategies at the same time, allowing
tests/invocation-overhead to try them "all at once" for side-by-
side comparisons.

Add support for a new JNIEnv invocation strategy which relies on
C#9 Function Pointers, a'la:

partial struct JNIEnv {
    public  delegate* unmanaged <IntPtr /* env */, jobject> ExceptionOccurred;
}
partial class JniEnvironment {
    partial class Exceptions {
        public static unsafe JniObjectReference ExceptionOccurred ()
        {
            IntPtr __env = JniEnvironment.EnvironmentPointer;
            var tmp = (*((JNIEnv**)__env))->ExceptionOccurred (__env);
            return new JniObjectReference (tmp, JniObjectReferenceType.Local);
        }
    }
}

This could allow for performance better than "JIPinvokeTiming",
as it avoids P/Invoke overheads to a set of java_interop_* C
functions (926e4bc), while also avoiding the overheads involved
with using Marshal.GetDelegateForFunctionPointer() as used by
"JIIntPtrs".

…but it doesn't necessarily provide better performance:

$ JI_JVM_PATH=$HOME/android-toolchain/jdk-11/lib/jli/libjli.dylib dotnet tests/invocation-overhead/bin/Debug/net6.0/invocation-overhead.dll
# SafeTiming timing: 00:00:04.2123508
#	Average Invocation: 0.00042123508ms
# XAIntPtrTiming timing: 00:00:02.1625501
#	Average Invocation: 0.00021625500999999998ms
# JIIntPtrTiming timing: 00:00:02.3620239
#	Average Invocation: 0.00023620239ms
# JIPinvokeTiming timing: 00:00:01.8993587
#	Average Invocation: 0.00018993587ms
# JIFunctionPointersTiming timing: 00:00:02.0278083
#	Average Invocation: 0.00020278083ms

(Compare and contrast with 926e4bc, circa 2015!)

Of particular note is that the Average Invocation time for
JIFunctionPointersTiming takes 7% longer than JIPinvokeTiming.

Though that's slightly reversed when a Release build of
invocation-overhead.dll is used:

% JI_JVM_PATH=$HOME/android-toolchain/jdk-11/lib/jli/libjli.dylib dotnet tests/invocation-overhead/bin/Release/net6.0/invocation-overhead.dll
# SafeTiming timing: 00:00:03.4128431
#	Average Invocation: 0.00034128431000000003ms
# XAIntPtrTiming timing: 00:00:01.8857456
#	Average Invocation: 0.00018857455999999999ms
# JIIntPtrTiming timing: 00:00:01.9075412
#	Average Invocation: 0.00019075412ms
# JIPinvokeTiming timing: 00:00:01.6993644
#	Average Invocation: 0.00016993643999999998ms
# JIFunctionPointersTiming timing: 00:00:01.6561349
#	Average Invocation: 0.00016561349ms

With a Release build, the Average Invocation time for
JIFunctionPointersTiming takes 97% of the time as JIPinvokeTiming,
i.e. is 3% faster.

We may or may not continue investigation of C#9 Function Pointers
for JNIEnv binding purposes. We will preserve this code for
future investigation.