emsha: embedded secure hashing

This library is an HMAC-SHA-256 Rust library designed for embedded systems. It is built following the JPL Power of Ten rules. It was written in response to a need for a standalone HMAC-SHA-256 package that could run on several platforms, including several memory- constrained embedded platforms. It works without using the Rust standard environment.

I had previously written a C++ version; porting it to Rust seemed a natural way to keep trying to learn the language.

Note: it requires the 2024 edition of Rust due to its use of core::error::Error.