[FEA] support for nextafter

[gmarkall]

The PR for this already exists in the Numba repo: numba/numba#9541

This issue is created as a placeholder / reminder to continue its development here.

cc @s-m-e - I realise this is asking for extra work on top of what the PR already did, so if you would prefer I can bring it over here - just let me know how you'd prefer to proceed.