Add option to preserve capacity when empty! is called on IdDict

[Zentrik]

I couldn't find a function for just emptying an IdDict without resizing it so I just added that as an option.

[KristofferC]

Unless this is part of the generic empty! function and is only applicable to IdDict I think it is better for this to be a separate function.

[Tokazama]

This should also apply to Dict

[Zentrik]

Interestingly, it seems empty!(a::Vector) doesn't resize

julia> b = collect(1:10^4);

julia> empty!(b)
Int64[]

julia> unsafe_load(convert(Ptr{Csize_t}, pointer_from_objref(b)+8*2+2+2+4+8)) |> Int
10000

Not sure why Dict and IdDict do in contrast.

Do you have any suggestions for a name, I was thinking zero!.

[Tokazama]

I assumed differences are due to how dictionaries resize using different approaches. I can't think of any collection that uses 'empty!' that wouldn't benefit from this extra argument. Not sure why it needs different name.

[vtjnash]

I think the observation was that an oversized Dict performs worse in real-world conditions than one that is resize! to fit the current contents. That doesn't apply to an oversized Vector.

[vtjnash]

c.f. e62a7db for measurements

[Tokazama]

I'm always for improving memory management tools but I'm curious what scenario one deletes all keys and values only to replace them with the same number of unique key value pairs

[Zentrik]

It's useful in #51120, essentially I have an IdDict which has a known max size and needs to be reset on every iteration.
So I want it to be preallocated for the max size to prevent rehashing on every iteration. So I want to empty it whilst persevering the size.