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

bytecode.h

@@ -25,6 +25,9 @@ enum {
   OP_HAS_UFUNC = 64,
   OP_IS_CALL_PSEUDO = 128,
   OP_HAS_BINDING = 1024,
+  // NOTE: Not actually part of any op -- a pseudo-op flag for special
+  //       handling of `break`.
+  OP_BIND_WILDCARD = 2048,
 };
 struct opcode_description {
   opcode op;