reconsider behavior of `collect_as` for empty input, regarding output element type and use of return type inference

[nsajko]

Current behavior:

julia> using FixedSizeArrays

julia> FixedSizeArrays.collect_as(FixedSizeVector, (n*n for n ∈ 1:0))
0-element FixedSizeVector{Any, Memory{Any}}

julia> collect((n*n for n ∈ 1:0))
Int64[]

We return an empty array with eltype of Any, while collect from Base returns an empty array with an element type of Int. Neither of these is ideal:

• Any is obviously suboptimal
• Int seems nice at first, but necessitates the use of return type inference. This is quite undesirable:
  • It inhibits compiler optimization AFAIK
    • in particular, the :foldable effect requires :nortcall
  • It makes the behavior depend on type inference, which makes for a terrible programming and debugging experience.

Unlike Base, we're not yet constrained by compatibility considerations, so I think we should:

• avoid making our behavior depend on type inference (in particular, I think this means not doing things like map(identity, some_array) in the implementation of collect_as)
• for empty input, we could do something like this:
  • set the output element type to Union{}, for an input element type equal to Any
  • set the output element type to the input element type, when it's not equal to Any

[oscardssmith]

this seems like the better default