Initial import and port from C++ code.

README.md

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+# 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](http://spinroot.com/gerard/pdf/P10.pdf) 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](https://git.wntrmute.dev/sc/emsha);
+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`.