Introduce function barrier in higher dimensional transform loop to avoid a lot of allocations.

[andreasnoack]

It's not a huge overhead but the number of allocations is reduced quite a bit with just a minor code change

Current main

julia> @btime fft(x) setup=(x=complex.(randn(256,256)));
  1.615 ms (5422 allocations: 2.14 MiB)

This PR

julia> @btime fft(x) setup=(x=complex.(randn(256,256)));
  1.402 ms (45 allocations: 2.00 MiB)

[andreasnoack]

I've added some allocation tests. Because of that, I have set the minimum version for Julia to 1.9. With 1.10 being lts, I don't think it is a big deal but let me know if you disagree and I can make the allocations conditional on the Julia version.

[dannys4]

See #64