Update osl_int

- Remove mpz_t* casts
- Add osl_int_size_in_base_2 function
- Add osl_int_floor_div*, osl_int_mod, osl_int_gcd and osl_int_size_in_base_10 functions
- Use const in osl_int* functions
- Add overflow detection
- Add osl_int test (in CMake only)