[JExtract] Add support for JNI member methods

[madsodgaard]

Adds support for generating code for member methods in JNI mode. Also fixes a bug with throwing functions, that also affected the JavaKit macros.

public func doSomething(x: Int64) {}

is generated as:

Java

 public void doSomething(long x) {
  long selfPointer = this.pointer();
  MyClass.$doSomething(x, selfPointer);
}

private static native void $doSomething(long x, long selfPointer);

Swift

@_cdecl("Java_com_example_swift_MyClass__00024doSomething__JJ")
func Java_com_example_swift_MyClass__00024doSomething__JJ(environment: UnsafeMutablePointer<JNIEnv?>!, thisClass: jclass, x: jlong, selfPointer: jlong) {
  let self$ = UnsafeMutablePointer<MyClass>(bitPattern: Int(Int64(fromJNI: selfPointer, in: environment!)))!
  self$.pointee.doSomething(x: Int64(fromJNI: x, in: environment!))
}

[ktoso]

Very good! just the nitpick about the passing in the thread. Let's fix that separately maybe :-)