Merge: Java FFI: add GC pinning support

Adds a missing features to the Java FFI: pinning references to Nit objects from Java. Both services, `..._incr_ref` and `..._decr_ref`, are "simple" redirections from Java to the nitni service in C through the JNI.

Pull-Request: #1820
Reviewed-by: Jean Privat <jean@pryen.org>