Disabled GC.

Broken for use with threads.
Revert "Changed call convention for threads."

This reverts commit 8d58e4b238fcd1d001384d683983afb585ebf76d.
Minor Makefile fix.

stdlib now gets compiled in default target.
Added "Copyable" type and impl.

In order to be sent via the actors.Actor.! method, a message needs to be
deep copyable (repr. by the Copyable type).

Implemented the copy method for a few primitives in the gracelib.