New Issue: [Library Stabilization] Implementation details involving casts and stability

17710, "lydia-duncan", "[Library Stabilization] Implementation details involving casts and stability", "2021-05-12T22:08:09Z"

We do a lot of casts to C types, but we use the GMP module. Should we be casting to its types instead where applicable? That seems more stable in case they change the type they represent under the covers (though it does seem unlikely that they will make changes like that frequently)