[Tracking] MultiDict and SortedMultiDict API updates before v1.0

I wanted to create this meta-issue to track all the API changes that we want to do for MultiDict, which are currently scattered around various other threads:

• PR https://github.com/JuliaCollections/DataStructures.jl/pull/656 (MuttsDict constructors operate only on elements/pairs, not on the aggregated underlying view)
• https://github.com/JuliaCollections/DataStructures.jl/issues/655 (Make iterate iterate over the pairs, not over the aggregated view)
  • WIP fix in https://github.com/JuliaCollections/DataStructures.jl/pull/676
• https://github.com/JuliaCollections/DataStructures.jl/issues/654 (Add delete!(d, k, v) to delete by pair)
• Exposition: https://github.com/JuliaCollections/DataStructures.jl/issues/522#issuecomment-675171633

And to use this thread to discuss other changes we might make.

Current suggested changes:

• Make MultiDict API consistently operate over pairs, just like a normal Dict, except that multiple entries are allowed per key.
  • WIP fix in #676
• Use a Set{T} instead of Vector{T} for the multi-values ([?] -- see discussion below)
  • WIP fix in #692

Open Questions:

• Should a MultiDict be <: AbstractDict?
• Should we include setindex!(md::MultiDict, k, v)? See https://github.com/JuliaCollections/DataStructures.jl/issues/666#issuecomment-716058978, below.

Start with the linked issues and pull requests, especially #654, #655, #656, #676, and #692, then read the exposition in #522. The checklist and open questions define the scope, but several items are still under discussion or have work in progress. Done would require resolving the agreed MultiDict and SortedMultiDict API changes and updating the related work accordingly.