base: make diff() use views and broadcasting

[stev47]

fixes #27957
fixes #15414 (didn't see this at the time of coding)

This uses array views and broadcasting to implement diff() in arbitrary dimension.
Added benefit is a significant performance increase for arrays with more than about 10 elements (at least in my tests).

x = rand(1000)
y = rand(1000,1000)
z = rand(10)
@btime diff($x);
@btime diff($y, dims=2);
@btime diff($z);

before:

919.474 ns (3 allocations: 7.98 KiB)
1.612 ms (4 allocations: 7.62 MiB)
41.718 ns (3 allocations: 208 bytes)

after:

504.201 ns (3 allocations: 8.03 KiB)
864.279 μs (4 allocations: 7.62 MiB)
41.598 ns (3 allocations: 256 bytes)

[timholy]

Very nice @stev47, just one tiny thing to fix and then I think this can be merged. There's some chance we'd need #28941 to solve this elegantly for arrays with offset axes, so I think it's fine to restrict this to 1-indexed arrays for now.

(@juliohm, this is the implementation I was hinting at with my comment about "in one go.")

[juliohm]

Thanks @stev47 for the addition, and thanks @timholy for pinging. 👍

[timholy]

Sorry for one more request, but I think the docstring needs updating too. Does this need a NEWS item?

[stev47]

updated the docstring