[ base ] Better set's toList implementation + preparation for cleanup

[buzden]

Description

This is mainly the less controversial part of #3392, reimplementing and deprecating completely useless specialised toList function from SortedSet.

Additionally, this PR contains some preparation to future removing the redundant Libraries.Data.Sorted{Set|Map} from the compiler's internal lib, the fact coming from times when Sorted{Set|Map} were in contrib. But currently, they cannot be replaced, because implementations in the compiler's lib have several functions that are missing in the implementation at base, so I suggest adding them there, while also doing the same thing with deprecating specialised toList, so that we just could remove those redundant modules once we've released new version of the compiler.

I made all changed in different commits in order to make review easier.

Should this change go in the CHANGELOG?

• If this is a fix, user-facing change, a compiler change, or a new paper
  implementation, I have updated CHANGELOG_NEXT.md (and potentially also
  CONTRIBUTORS.md).