Add a SortedSeq collection type

[neontorrent]

This is a new proposal for a previously closed issue: scala/collection-strawman#132

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Use case:

In the past we had CanBuildFrom that we can pass to map/collect/etc. methods, and turn the collection into something else. I personally use coll.map[B, That] {/* lambda */}(new CanBuildFrom[Coll, B, That] {/* impl */}).sortedBy(_.intField) to sort any Iterable into a sorted Seq.

This is a better performant version than coll.map{/**/}.toSeq.sortedBy(_.intField) as it does not create an intermediate collection Seq

However after 2.13, we no longer have CanBuildFrom to pass into map/collect/etc., and there is no alternative to convert an Iterable to a sorted collection without creating an intermediate Seq.

Proposal:

Since CanBuildFrom is gone for good, I propose we should have a SortedSeq so that the use case above can be achieved with less performance penalty. Then we can do:

val coll: Iterable[String] = List("100", "11")
coll.to(SortedSeq.evidenceIterableFactory(Ordering.by((_: String).toInt)))

This allows duplicate items in the collection, which SortedSet cannot do.

An example of possible implementation can be found at: https://contributors.scala-lang.org/t/new-collection-sortedlist/314/21