[ new ] added util functions for SortedMap

[claudio-etterli]

Description

Added insertWith, insertFromWith and fromListWith to the SortedMap module.
The idea is to insert key-value pairs and decide how duplicated values should be merged.
Unique key-values pairs will be appended to the SortedMap.

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).

[gallais]

that's a direct corollary of update so not sure it's needed?

[stefan-hoeck]

I had a look at Haskell's Data.Map today, and I am in strong favour of adding the utilities from this PR (including insertWith). The Haskell module has a function like Data.SortedMap.update (it's called alter there) as well as insertWith, fromListWith, and several other related utilities.

In my own code, the need for insertWith comes up quite frequently, while I hardly ever used update so far, probably because it is much more comon to add something to a SortedMap than to delete stuff.