Add support for the quotient ring ZZ/n when n is large or composite

[d-torrance]

We currently use FLINT for quotient rings created by ZZ/n and thus require n to be prime and to fit inside an unsigned int:

i1 : ZZ/24
stdio:1:3:(3): error: ZZ/n not implemented yet for composite n = 24

i2 : ZZ/nextPrime 2^100
stdio:2:3:(3): error: expected argument 1 to be a small integer

But the engine handles these things just fine if we create a quotient ring of a polynomial ring with no variables, as suggested in #272:

i1 : ZZ/24

     ZZ[]
o1 = ----
      24

o1 : QuotientRing

i2 : ZZ/nextPrime 2^100

                   ZZ[]
o2 = -------------------------------
     1267650600228229401496703205653

o2 : QuotientRing

Closes: #272