Add `label $name | EXP`

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

Still needed:

 - docs
 - testing
 - syntactic sugar for `$break|error` ?  Nah, instead we could have the
   `break` builtin check if its input looks like a break label, then error
   that, else `"break"|error` for backwards compat.