Integer: GMP: Move implementation of member functions to .cpp file. (#5190)

Integer: GMP: Move implementation of member functions to .cpp file. (#5190)

This moves the GMP implementation of member functions of class Integer from
the header to the .cpp file. This only moves code, and adds documentation for
previously undocumented or poorly documented functions.

I ran a performance check on the cluster on non-incremental QF_BV, QF_BVFP,
QF_FP, QF_LIA, QF_LIRA and QF_LRA (BV and FP logics heavily rely on class
Integer since class BitVector is implemented on top of Integer). This change has
no impact on performance.