Builds upon the vanilla Scala parser and teams up together with Placeholders.scala to emulate holes.
A principled solution to splicing into Scala syntax would be a parser that natively supports holes.
Unfortunately, that's outside of our reach in Scala 2.11, so we have to emulate.
Wrapper around tree parsed in q"..." quote.
Wrapper around tree parsed in q"..." quote. Needed to support ..$ splicing on top-level.