Implement labels, goto, labeled break, and labeled continue.
Return checking is now done as a general flow check at the end
of function compilation, since break and goto complicated the
way I was doing return checking before. Goto-over-declaration
checking is also done as a final flow check.
Temporary variables used for effect extraction are now
actually temporary. Otherwise "op=", "++", and "--" appear as
declarations that cannot be jumped over.