RFC: IdDict{K,V} as wrapped ObjectIdDict

[mauro3]

This adds an IdDict{K,V} which uses === and object_id for equality and hashing as is needed for #24354. It is implemented by wrapping the good old ObjectIdDict and using type assertions on the getters/setters. This is an alternate implementation of #24932, as was suggested by @JeffBezanson in #24932 (comment). Unlike #24932, this PR passes all tests locally and should be mostly ready to go.

This PR deprecates the "old" ObjectIdDict in favor of the new IdDict. To make this work I had to rename ObjectIdDict to something else: _ObjectIdDict. Note though that _ObjectIdDict is still used and needed in the core of Julia.

Performance of the IdDict is a bit worse (25%) than for ObjectIdDict, presumably because of the extra layer, as the somewhat rudimentary tests in the gist show.

RFCs:

• is the rename of ObjectIdDict necessary or could it be deprecated without rename?
• One thing I am not sure about is the, now updated, following statement in the docs: "Alternatively, you can use the IdDict dictionary type, which is specially handled by precompilation so that it is safe to initialize at compile-time." Is this still true? If not it probably should be made true. This needs checking by someone!

This should probably be squashed before merging.

[JeffBezanson]

It seems to me _ObjectIdDict should be unnecessary. What goes wrong without it?

[mauro3]

Not renaming (going back to 7d4908e) and adding to deprecated.jl:

export ObjectIdDict
const _dep_message_ObjectIdDict = ", use IdDict{Any, Any} instead"
deprecate(Base, :ObjectIdDict)

compiles but throws dep-warns from the (legitimate) usages in Base. Can you point me to how you would go about it? One option could be to only have ObjectIdDict in Core, but I don't know how that works or whether that is a good strategy. So, I need a bit of guidance for this.

[JeffBezanson]

The uses in Base can use IdDict{Any,Any} directly, or we could provide the ObjectIdDict alias without deprecation.

[mauro3]

I changed the usages of ObjectIdDict which are after dict.jl in the boot-strap to IdDict, but the ones before (in Core) still use ObjectIdDict. To move the Core-uses to IdDict is beyond my skills.

I think it would be nice to deprecate ObjectIdDict, otherwise we provide two types with almost the same use, leading to function signatures like f(d::Union{IdDict,ObjectIdDict}) = .... One strategy could be to rename & deprecate in 0.7 and in 1.0 rename again to ObjectIdDict but not export anymore (to get rid of the ugly _ObjectIdDict name).

[JeffBezanson]

Replaced by #25210