Accept GMP|string|int union in GMP functions

[nikic]

This changes GMP functions to accept a GMP|string|int union with standard semantics (and thus also uses it in function signatures).

Relative to the previous behavior, this means that GMP functions in weak mode now also accept float and null (con), and in strict mode no longer accept bool (pro), and have full type information (pro).

@kocsismate What do you think about this?

[kocsismate]

@nikic I think it's a totally reasonable change, and IMO the pros outweigh the cons :) Especially, because I wouldn't expect people using strict mode to pass bool values to these functions, so the BC impact should also be negligible.