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

lexer.l

@@ -57,6 +57,10 @@ struct lexer_param;
 "//" { return DEFINEDOR; }
 "try" { return TRY; }
 "catch" { return CATCH; }
+"label" { return LABEL; }
+"break" { return BREAK; }
+"break2" { return BREAK2; }
+"break3" { return BREAK3; }
 "|=" { return SETPIPE; }
 "+=" { return SETPLUS; }
 "-=" { return SETMINUS; }