Numba linker

This adds a `numba` non-recursive linker, plus tests for it. All linking tests pass with and without `numba` present. The user can select 'numba' as a `link_strategy`, or use the 'auto' option which can fall back to 'recursive'.

I have not yet evaluated performance but would like to make this available for testing now, because... it's a busy week for me, and I don't know if I'll be able to do that until next week.