In developing the Rust wrappers for libmongocrypt, we ran into the issue that many functions take "morally const" arguments (mongocrypt_binary_t as input parameters, mongocrypt_ctx on accessor methods like mongocrypt_ctx_state, etc.). Ideally, those could reflect that quality by taking those arguments as const pointers; this would be a useful bit of additional inline documentation, and would allow us to be more confident in the safety of the Rust wrapper.