preserve immutable collections in to(Target) conversions

[lrytz]

In Scala 2.13, collection.to(Target) conversions return the collection object unchanged if it is immutable and is already a subtype of the Target type.

This PR brings the same behavior to Scala 2.12 when using scala-collection-compat.

The 2.12 codebase is unchanged, so a to[Target] conversion may still create unnecessary copies.

Remaining differences between 2.12 and 2.13:

• static types: 2.12's to is defined as def to[Col[_]](implicit cbf: CanBuildFrom[Nothing, A, Col[A]]): Col[A], so to(Map) has static type Iterable[(K, V)], not Map[K, V]

• lazy maps: mapValues returns a lazy map that's a subtype of immutable.Map, so

  scala> val mv = Map(1 -> 1).mapValues(_ + 1)
  scala> mv.to(Map) eq mv
  res7: Boolean = true // false in 2.13, a strict copy is created
  

[lrytz]

review by @retronym

[lrytz]

copy paste from above, not sure how to factor things out in sbt

[lrytz]

cc @martijnhoekstra, since you originally wrote IdentityPreservingBuilder for #238. This PR applies it more widely.

[SethTisue]

this might interest @scala/collections

[NthPortal]

initial review is that it looks correct. if I get more energy, hopefully I can do a more thorough review.

I did not review of the sbt changes, as I'm less confident in my ability to refactor sbt configs than in yours

[lrytz]

Thank you @NthPortal!

[SethTisue]

@lrytz can you fill in the PR description?

[lrytz]

👍 done

[NthPortal]

oh, this is already released, awesome!