Clearer view of what a Unicode lexer can do.
Change records & maps syntax.

Related to PRs #29 & #30.

* Added a '#' as prefix
* Simplified empty record & map and record type
* Removed `kv`: no need now, ambiguities…
* Add '#' to `mc`: clearer, also can use seqExprs there
* Had to patch `Comment` to allow '#{' stuff
* Remove unused tokens
* Update tests