Indexing performance + lazy dense dicts

[joaquimg]

Follows: #1129

This avoids the conversion from dense dict to standard dict in copy functions.

[blegat]

I remember we hesitated between adding a dict in DenseDict or using DenseDict inside a CleverDict to switch from DenseDict to Dict in case of deletion. I thought we decided the second one but I don't remember the details

[blegat]

I think we should

1. replace the vector by a DenseDict in CleverDict
2. add the possibility to add a new contiguous index to DenseDict
3. implement setindex! for CleverDict similarly to this PR

[joaquimg]

I remember we hesitated between adding a dict in DenseDict or using DenseDict inside a CleverDict to switch from DenseDict to Dict in case of deletion. I thought we decided the second one but I don't remember the details

I think we only hesitated because it could be too similar to CleverDict.

[joaquimg]

We need to be careful with CleverDict because of the downstream usage.
We need to check if theses changes will be breaking.

[joaquimg]

one problem is that:
CleverDicts requires the methods index_to_key and key_to_index

MathOptInterface.jl/src/Utilities/CleverDicts.jl

Line 9 in 746b2e1

function index_to_key(::Type{MathOptInterface.VariableIndex}, index::Int)

As methods to be overloaded, while DenseDicts get hash and inverse_hash in the type constructor.
This would be breaking for solvers like GLPK that overload the CleverDict methods:

https://github.com/jump-dev/GLPK.jl/blob/41e649cd81864e90065ac78f42317edcebcd9951/src/MOI_wrapper/MOI_wrapper.jl#L34

[blegat]

We could use these functions as default for the map and inverse map of DenseDict so that it is non-breaking

[odow]

It is a bit confusing that we have so many types of dictionaries. It's probably okay to get some performance improvement for now, but at some point I'd like to go through and tidy up a lot of this.

[joaquimg]

So instead of adding a DenseDict inside CleverDict I will merge the two of them.
As @blegat that can be done without breaking. And DenseDict was never released anyway.

[blegat]

Merging them sounds good

[blegat]

@joaquimg Is this closed by #1142 ?