Updating expressions syntaxes in CLS

Featuring:
+ new keyword `label` (will be used as an alternative with `goto` to labels `<label>`)
+ loops syntax, incl. for-each, while/until, loop (;;)
+ generators, incl. yielding {generators} and looping (generators)
+ collection comprehension syntax (finally solved!)
- removed keyword `undef`, possibly will not ever need it since Coral got `abstract` and stuff