math.nextafter for cuda

[s-m-e]

This PR adds support for math.nextafter for CUDA. Partially fixes #9435 (no support for numpy.nextafter), related to #9424 and #9438.

[s-m-e]

Caveat: This PR intentionally only implements support for FP32 and FP64, but not FP16. I failed to figure out how to do this correctly and do not require it for my use case at the moment.

[s-m-e]

@esc It appears I forgot the release notes - please re-run CI.

[s-m-e]

Ping @gmarkall

[gmarkall]

Looks like there is a fast version of nextafter for 64-bit operands too: https://docs.nvidia.com/cuda/libdevice-users-guide/__nv_nextafter.html#__nv_nextafter - I'm not 100% sure this will work (been a while since I thought about these implementations) but maybe:

Suggested change

	binarys_fastmath['nextafterf'] = 'fast_nextafterf'
	binarys_fastmath['nextafterf'] = 'fast_nextafterf'
	binarys_fastmath['nextafter'] = 'fast_nextafter'

[gmarkall]

Additionally - if there are fastmath implementations of these functions, they should also be tested in numba/cuda/tests/cudapy/test_fastmath.py - you should be able to create a test by following the pattern used in the tests for other functions. If you're having trouble working out what patterns to check for, let me know and I'll see if I can help work out something appropriate.

[gmarkall]

Thanks for the PR! On the whole this looks good, I just have a comment about the fast implementation and testing it on the diff.

I think omitting float16 support for now is fine - I didn't see an implementation in the cuda_fp16.{h,hpp} headers. I did see something that may be applicable in libcu++, but the route to using it (or whether it is applicable) is not obvious so I'm happy with leaving it for now.