Embed offset in new-style tokens

Parser now consumes an `Iterable[Tok]` and that's it. 

review @xeno-by 
If I name class as just `_` it fails to compile for some weird reason:
```
[error] /Users/Denys/Proj/meta/scalameta/syntactic/parsers/Tok.scala:101: class Any is abstract; cannot be instantiated
[error]   case class `_`(offset: Int) extends Delim with ExprIntro with TypeIntro
[error]              ^
```
Probably another name mangling bug.