On Rust 1.81, abort by unwinding past extern "C"

In Rust 1.81, unwinding past an extern "C" is guaranteed to produce an abort. This is preferable to the previous fallback behavior of triggering a double panic because it doesn't produce a backtrace.

On previous versions, this functionality is disabled because it triggers undefined behavior.

This only affects the "fallback" behavior when there is no other implementation of abort.

Decrease minimum supported rust version to 1.31.
Use Github Actions for automated testing.