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

compile.h

@@ -50,7 +50,9 @@ block gen_or(block a, block b);
 block gen_var_binding(block var, const char* name, block body);
 
 block gen_cond(block cond, block iftrue, block iffalse);
+block gen_try_handler(block handler);
 block gen_try(block exp, block handler);
+block gen_label(const char *label, block exp);
 
 block gen_cbinding(const struct cfunction* functions, int nfunctions, block b);