Implement Java Bytecode Generator #11922

Context: Issue dotnet/java-interop#524
Context: Issue dotnet/java-interop#534

Allowing JVM-hosted code to invoke managed code requires the use of Java Callable Wrappers for all Java.Lang.Object subclasses. The Java Callable Wrapper contains Java native method declarations for all methods implemented in managed code, e.g. by overriding a virtual method, implementing an interface method, or placing [Java.Interop.ExportAttribute] on a method.

Java Callable Wrappers are currently implemented by using Java source code as an intermediary: JavaCallableWrapperGenerator generates Java source code, which is later compiled by javac to produce Java byte code for later inclusion into an Android application.

We have historically pondered updating Xamarin.Android.Tools.Bytecode to be a read/write API instead of a read-only API, largely for performance reasons: we could avoid the overhead of writing .java files and invoking javac if we just directly emitted a .jar file containing relevant .class files.

We now have another reason to ponder this idea: Full Kotlin support. Kotlin "name mangles" method names in various contexts, emitting Java byte code method names which are not valid Java identifiers:

// Kotlin
package example;

public open abstract class AbstractUnsigned {
	public abstract fun m(value : UInt)
}

// javap example.AbstractUnsigned
public abstract class example.AbstractUnsigned {
  public abstract void m-WZ4Q5Ns(int);
  public example.AbstractUnsigned();
}

Note the example.AbstractUnsigned.m-WZ4Q5Ns() method: it cannot be overridden from Java:

// error: JavaSubclassAbstractUnsigned is not abstract and does not override abstract method m-WZ4Q5Ns(int) in AbstractUnsigned
class JavaSubclassAbstractUnsigned extends example.AbstractUnsigned {
}

class JavaSubclassAbstractUnsigned extends example.AbstractUnsigned {
	public void  m-WZ4Q5Ns(int value) {		
// error: '(' expected
//	public void  m-WZ4Q5Ns(int value) {		
//	              ^
// error: invalid method declaration; return type required
//	public void  m-WZ4Q5Ns(int value) {		
//	               ^
	}
}

So long as we use Java source code as an intermediary, we cannot override or implement Kotlin name-mangled methods.

We could use Kotlin as an intermediate language ("Kotlin Callable Wrappers," anyone?)...

...but why add Kotlin as an intermediate language when we could instead just write Java byte code directly. This would allow the Java Callable Wrapper process to be faster -- no javac invocation would be required -- and means that we avoid any "pesky" issues caused by Kotlin name mangling.