Make better use of ExceptionHandling.dieInternal() in NativeCallOps.
Better commenting of code-gen in CStruct.java.
Implement NativeCall callbacks on JVM.

This passes the NativeCall test suite, so it should be pretty functional. No
doubt there are bugs lurking though.

With this, the only missing pieces for NativeCall on JVM are CArray of
pointer-valued classes, and the nqp::nativecallrefresh op.