Indexing arrays with AbstractUnitRanges retains indices

[jishnub]

In v1.6

julia> using OffsetArrays

julia> ones(10)[Base.IdentityUnitRange(2:3)]
2-element OffsetArray(::Vector{Float64}, 2:3) with eltype Float64 with indices 2:3:
 1.0
 1.0

On master:

julia> ones(10)[Base.IdentityUnitRange(2:3)]
2-element Vector{Float64}:
 1.0
 1.0

This happens because #39896 changed the behavior of getindex, and it now uses the length of the indices instead of the axes to generate the output array. This PR reverts this to make the result account for the axes again. After this:

julia> ones(10)[Base.IdentityUnitRange(2:3)]
2-element OffsetArray(::Vector{Float64}, 2:3) with eltype Float64 with indices 2:3:
 1.0
 1.0

Also,

on master

julia> using StaticArrays

julia> ones(10)[SOneTo(2)]
2-element Vector{Float64}:
 1.0
 1.0

After this PR (back to v1.6 behavior):

julia> ones(10)[SOneTo(2)]
2-element SizedVector{2, Float64, Vector{Float64}} with indices SOneTo(2):
 1.0
 1.0

The performance impact on changing unsafe_copyto! to copyto! appears to be minimal.

with copyto!

julia> a = ones(20000);

julia> @btime $a[$ax];
  19.084 μs (2 allocations: 156.33 KiB)

with unsafe_copyto!

julia> @btime $a[$ax];
  18.968 μs (2 allocations: 156.33 KiB)

The difference is within the noise level.