Dispatch on traits in iterate for AbstractArrays and types that wrap AbstractArrays

[jishnub]

An attempt at using traits in iterate for AbstractArrays (Fixes #41945), which makes iteration for wrapper types (eg. OffsetArrays) use the same iterate method used for the parent array, consequently obtaining the same performance. Array wrapper types may opt-in to the iteration behavior of the parent through the trait IterationStyle, for example as

Base.IterationStyle(A::OffsetArray) = Base.IterationStyle(parent(A))

By default they will fall back to the iterate method for AbstractArrays, which is the present behavior.

A comparison of the performance without using the trait and with it (see #41945 for other examples):

Without defining the trait for OffsetArrays:

julia> r = 1:1:1000; ro = OffsetArray(r);

julia> @btime Float64[i for i in $r];
  1.621 μs (1 allocation: 7.94 KiB)

julia> @btime Float64[i for i in $ro];
  25.306 μs (1 allocation: 7.94 KiB)

With the trait defined:

julia> @btime Float64[i for i in $r];
  1.677 μs (1 allocation: 7.94 KiB)

julia> @btime Float64[i for i in $ro];
  1.609 μs (1 allocation: 7.94 KiB)

I would love some feedback on this as it is a somewhat large change, although there seems to be a significant performance benefit to this.

[timholy]

I like this, although since iterate relies on indexing your tests might also check OffsetArray with an offset other than 0; iterate(OffsetArray(v, (0,)) and iterate(OffsetArray(v, (1,))) should yield the same sequence of values (all the elements of v).

My main response is to consider whether we can be a bit more indirect. From profiling, to me it seems that bounds-checking accounts for an awful lot of the performance gap. Intervening there, rather than directly in iterate, thus seems likely to benefit a wider range of use cases. #42029 and #42030 are efforts in that direction. Shall we see how that pans out and then consider this later? I'll drop my objection to JuliaArrays/OffsetArrays.jl#257 in the meantime, since I recognize that the path I'm proposing may take a while.

[jishnub]

Fair enough, that seems like a reasonable approach. Best to narrow down the issue before introducing more traits.