`ArchetypeAccess` was tracking `immutable` and `mutable` separately.
This means that checking is_compatible requires three checks:
m+m, m+i, i+m.
Instead, continue tracking `mutable` accesses, but instead of
`immutable` track `immutable | mutable` as another `accessed` bit mask.
This drops the comparisons to two (m+a, a+m) and turns out to be
what the rest of the code base wants too, unifying various duplicated
checks and loops.
