moves the impl of quasiquotes to scala.reflect

This brings consistency with scala.reflect.reify and scala.reflect.macros
already existing in scala-compiler. To the contrast, scala.tools.reflect,
the previous home of quasiquotes, is a grab bag of various stuff without
any central theme.
Merge pull request #3846 from xeno-by/topic/move-quasiquotes

moves the impl of quasiquotes to scala.reflect