Avoid Rust null checks for C++ non-null pointers

[teor2345]

When a pointer (or reference) is guaranteed to be non-null in C++, it should interop with Rust NonNull, ideally without redundant null checks.

Crubit's solution is here: https://crubit.rs/design/design.html#source-of-additional-information-that-customizes-c-api-projection-into-rust