Slight refactor to collection widening

[bramtayl]

Pull out specifically collection widening to its own functions. This will allow overloading these functions for data-structures which are easier to widen than Arrays, e.g. ModelArrays (see #13942)
The eltype checks seem redundant but help make the functions more generalizable. I think they're costless anyway.

[vtjnash]

This might need an explicit inline annotation, since part of the existing trick is that this elides dispatch by moving all of the dynamic cost into one branch

[bramtayl]

Ok

[bramtayl]

Wait, I'm confused.... adding the @inline annotations made CI fail? Did I make a typo or something?

[martinholters]

IIRC, @inline is not defined when array.jl is read during bootstrap. Try @_inline_meta (inside the function body) instead.

[bramtayl]

Oh ok

[bramtayl]

Well it looks like this is passing. Is it worth running benchmarks?

[andyferris]

I like this.

I hope this pattern will be easier to replicate in custom array code and custom operations on containers. Currently I find implementing a “widening” approach instead of a “inference” approach to container allocation quite tiresome, but I think this could help immensely.

[bramtayl]

Ok I think I've addressed all the comments

[bramtayl]

Good?

[andyferris]

Looks both sound and useful.

[bramtayl]

Is this backsportable?

[fredrikekre]

No, its not a bugfix.