let a dependency package own `collect_as`?

[nsajko]

I think it'd be slightly nicer if we let another package (https://github.com/JuliaArrays/CollectAs.jl?) own the collect_as function. That way:

• third-party packages might want to extend collect_as for their own collection types
• we wouldn't need to worry about whether to export collect_as or not, if a user wants it, they can do using CollectAs

The FixedSizeArray-specific methods would still be defined here.

[nsajko]

The CollectAs package on its own would presumably have methods for:

• Union{}: just throw
• Array: implemented using collect

[giordano]

Just to clarify: downstream packages would add methods via extensions?

[giordano]

Based on JuliaLang/julia#58513, do we actually need a new function or could extend collect (by wrapping the iterator in a new struct?)?

[nsajko]

I guess we could do both? Personally I think I prefer the collect_as approach, because of the different semantics regarding how it avoids relying on type inference to determine the element type. Also, it seems like collect_as might perhaps be useful for the ecosystem more widely, as any collection type could add methods to collect_as. BTW I'm working on CollectAs.jl currently.