Export WithId in the Rust compiler and use it in rust-sgx

[vn971]

Currently, our codebase uses transmute to handle "internal" and "external" definitions of WithId in Rust standard library (the SGX platform of it) and in rust-sgx (this repository). While technically currently "safe", we're lacking compiler-enforced checks on this type. It is arguably safer to rather export WithId and re-use it, thus avoiding the need for std::mem::transmute.

This was originally raised here: #515 (comment)