Standard library: MutableCollections

[robin-aws]

Mutable versions of common collection types.

Depends on #4640, since the current approach is using {:extern} to use efficient native implementations.

Update: starting with Vector from the existing JSON library, since it is pure Dafny that doesn't need extern code

Existing source:

dafny-lang/libraries/src/MutableMap (partial) - Paused since the Go and JS backends don't support hashcodes on all Dafny types, so it's not possible to implement efficiently yet
dafny-lang/libraries/src/JSON/Utils/Vectors.dfy

[robin-aws]

Note that the current implementation's example makes use of a generic AssertAndExpect utility that should be promoted somewhere more central.