Add GMP support to ForeignFunctions

[d-torrance]

This adds a new mpzT type to the ForeignFunctions package for working with GMP integers. For example:

i1 : needsPackage "ForeignFunctions"

o1 = ForeignFunctions

o1 : Package

i2 : mpzAdd = foreignFunction("__gmpz_add", void, {mpzT, mpzT, mpzT})

o2 = __gmpz_add

o2 : ForeignFunction

i3 : x = mpzT 0

o3 = 0

o3 : ForeignObject of type mpz_t

i4 : mpzAdd(x, 2, 3)

i5 : x

o5 = 5

o5 : ForeignObject of type mpz_t

i6 : value oo

o6 = 5