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