[ffigen] Context field for ObjC blocks/protocols

[liamappelbe]

ObjC implementation of jnigen's context proposal for interface implementation. The idea is that this context map would be stored on the native side as a map of weak references, to break Dart <-> Java/ObjC reference cycles that cause memory leaks.

Java doesn't have lambdas, so they only have to worry about interface implementation. But in the ObjC case we'll need to apply this to both blocks and protocols.

[liamappelbe]

Related: #1510