Support Integer, Z, and Rational in FFI

[qsctr]

Add support for Integer, Z, and Rational in the FFI, mapping to GMP mpz_t and mpq_t.

For Integer and Z arguments are mapped to const mpz_t input parameter and return types are mapped to mpz_t output parameter. For Rational it is const mpq_t and mpq_t respectively. Sequences of these types are supported too, mapping to pointers to those types.

Note: Right now Z always uses mpz_t even if the modulus is small enough to fit in an Int. Also, the type argument of Z is allowed to be polymorphic. Not sure if either of those are the right choices.

Remaining work:

• Add tests
• Update RefMan
• When generate headers code is merged, update it to support the new types

Closes #1395.