(squashed) M6 bugfixing spree

see c5681839ee for detailed history

Conflicts:
	quasiquotes/src/main/scala/scala/quasiquotes/QuasiquoteCompat.scala
	quasiquotes/src/main/scala/scala/quasiquotes/package.scala
	tests/src/test/scala/quasiquotes/TypecheckedProps.scala