[WIP] OrderedGeneralDict

[eulerkochy]

This is a modified version of #39 .

Work remaining --
I have to fix the OrderedSet tests, which broke because of this change

[vtjnash]

Wouldn't this be quite inefficient, with the need to store the keys twice, and to double the rehashing logic?

[PallHaraldsson]

I was benchmarking this and see it's incomplete:

(@v1.6) pkg> add OrderedCollections#06cc1db


(20/100) benchmarking "empty! OrderedGeneralDict() 10^4 elem Int64"...
[..]
ERROR: MethodError: no method matching (OrderedGeneralDict{Float64, Float64, DT} where DT<:AbstractDict{Float64, Int32})(::Vector{Pair{Float64, Float64}})

As I'm running late for including in Julia 1.6 and just discovered this new one, I'm probably going to skip it. Unless you know it should be much better. Any idea how incomplete it is? We could always change the ordered dict implementation in Base later, after guarantee in, and I'm conflicted which to start with.