Creates a GMP number from an integer or string. String
representation can be decimal or hexadecimal. In the latter case,
the string should start with 0x.
Note:
It is not necessary to call this function if you want to use
integer or string in place of GMP number in GMP functions, like
gmp_add(). Function arguments are
automatically converted to GMP numbers, if such conversion is
possible and needed, using the same rules as
gmp_init().
