Memoize ZZ/n for non-prime n too

[d-torrance]

We currently memoize ZZ/n in the savedQuotients hash table when n is prime, but we don't when it isn't (or is large), causing #4486.

Before

i1 : unique apply(20,i->promote(i,ZZ/4))

o1 = {0, 1, 2, -1, 0, 1, 2, -1, 0, 1, 2, -1, 0, 1, 2, -1, 0, 1, 2, -1}

o1 : List

Every element of this list belongs to a distinct ring!

After

i1 : unique apply(20,i->promote(i,ZZ/4))

o1 = {0, 1, 2, -1}

o1 : List

AI Disclosure

No AI used