SI-6591 Reify and path-dependent types

Slightly changed reification logic. Reified free values are marked as stable. Also:

test `reify_newimpl_30.scala` disabled due to [SI-7082](https://issues.scala-lang.org/browse/SI-7082)
test `t6591_4.scala` moved to pending due to [SI-7083](https://issues.scala-lang.org/browse/SI-7083)

review by: @xeno-by