Full FFI + Tests

[codefrau]

So far, SqueakJS implements only the subset of FFI calls and types needed to make codefrau/jasmine work.

It's reasonable to add more as the need arises, but it would also be useful to implement the full FFI interface. The best way to do that would probably be trying to make the FFI-Tests work (and potentially FFI-PoolsTests and FFI-CallbacksTests too).

This requires adding some more FFI primitives, but most importantly adding JS implementations of the functions provided by platforms/Cross/plugins/SqueakFFIPrims