Instructions now take their static arguments in the bytecode directly
instead of from the stack. The return address is now moved into a
separate frame stack. That means the only thing in the main stack is now
variable references. That means we can mark every variable which is
currently on the stack during a GC; even variables which aren't referred
to from a namespace.

As a result, a GC can happen before or after any instruction without
having to worry. The VM should also be faster, and the bytecode should
be smaller, due to fewer instructions.