Introduce support for Unliftable into Quasiquotes

Unliftable is a type class similar to existing Liftable that lets
users to extract custom data types out of trees with the help of
straightforward type ascription syntax:

  val q“foo.bar(${baz: Baz})” = ...

This will use Unliftable[Baz] to extract custom data type Baz out of
a tree nested inside of the another tree. A simpler example would be
extracting of constant values:

  val q”${x: Int} + ${y: Int}” = q”1 + 2”