`copyto!` sometimes copies whole source array when there is possibility of aliasing

[LilithHafner]

julia> using OffsetArrays, BenchmarkTools

julia> function _copyto!(dest, desto, src, srco, n)
           for i in 0:n-1
               dest[i + desto] = src[i + srco]
           end
           dest
       end
_copyto! (generic function with 1 method)

julia> x = rand(100_000);

julia> y = OffsetArray(x, 0);

julia> @btime copyto!(x, 20, x, 10, 5);
  20.728 ns (0 allocations: 0 bytes)

julia> @btime _copyto!(x, 20, x, 10, 5);
  18.036 ns (0 allocations: 0 bytes)

julia> @btime copyto!(y, 20, y, 10, 5);
  11.875 μs (2 allocations: 781.30 KiB) # That's microseconds, not nanoseconds!

julia> @btime _copyto!(y, 20, y, 10, 5);
  17.827 ns (0 allocations: 0 bytes)

The issue is that unaliasing checks for possible aliasing, and if present copies the entire source array. This causes major performance losses here. It can also cause bugs when reading from an array has side effects (this is responsible for a test failure in JuliaCollections/SortingAlgorithms.jl#71)

[KristofferC]

Retitled this since a number like 500x is a bit meaningless when it can be made into whatever number desired by changing the inputs.