Branching finally works, whew.

Now gets through another couple of function calls, and crashes
on hitting the unimplemented 'getmemsize' instruction.

I'm initializing stack frames but not tearing them down. Probably
best to compile each routine into two Lua functions, one to set
up and tear down the stack, the other to do the actual work. That
way it's always safe for Instruction:toCode() to emit a simple
"return" statement.