With this commit, `erlang_nif-sys` users can mark NIFs `unsafe`.
With this commit, `erlang_nif-sys` users can mark NIFs `unsafe`.

Happily, the changes are backwards-compatible, because any time Rust
expects an `unsafe` function, it's OK to pass a safe function instead.

goertzenator/ruster_unsafe,goertzenator/ruster_unsafe,hansihe/Rustler