numba cache and fastmath regression hardening #1805

Hazards from #1789 that can be done in parallel with other sub-issues:

H1: Cross-file stale numba cache via inline="always". numba validates a cache entry against (st_mtime, st_size) of the defining file only (caching.py:166-173). intersections.*.nbi is checked against intersections.py

H2: No occurences now, but it's possible that using fastmath inappropriately could destroy accuracy gains from compensated arithmetic.