Add label $name | EXP; fix break

This is to fix the problem where `break` is dynamic, not lexical.

With this it should be possible to do this sort of thing:

    label $break | inputs | if ... then $break|error else . end

This is a backwards-incompatible change for master, but the previous
`break` hadn't shipped yet.

Still needed:

 - testing

opcode_list.h

@@ -40,3 +40,4 @@ OP(TOP, NONE, 0, 0)
 OP(CLOSURE_PARAM_REGULAR, DEFINITION, 0, 0)
 OP(DEPS, CONSTANT, 0, 0)
 OP(MODULEMETA, CONSTANT, 0, 0)
+OP(GENLABEL, NONE, 0, 1)