Move typing of quotes and splices into its own trait

[odersky]

The code dealing with typing quotes and splices is 320 loc, about 10% of Typer as a whole. We want avoid Typer becoming excessively long. That's why I think it would be a good idea to move
this code into its own trait, inherited by Typer, similarly to what we did with Applications, Inferencing or Implicits.

[nicolasstucki]

Fixed by #6875