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().