Explicit type annotations for implicit definitions

[olafurpg]

Dotty requires implicit type annotations for implicit values. This means scalafix can help by rewriting code like this:

// before
implicit val x = 1
// after
implicit val x: Int = 2

To implement this rewrite, we need the scalac inferred types for bodies of Defn.{Val,Def} that contain the implicitmodifier.

[olafurpg]

I guess this can be closed now, first semantic rewrite wow :)