equix, hashx: Benchmark against C implementation

This is a small batch of improvements for the equix and hashx benchmarks. The headline feature is that we are now including the C implementations (slightly modified from tevador's, hosted as part of c-tor) and using them in apples-to-apples comparisons.

Minor features:

• Benchmarks moved to new nested crates, preventing their dependencies from spilling into the main workspace build.
• Tests are now grouped
• We also test the performance of memory reuse where possible
• Code cleanup for per-runtime options

These benchmark builds will now automatically pull in the c-tor git repo and build portions of it with a Rust wrapper. This uses the 'cc' and 'bindgen' crates, so it requires a C compiler and libclang on the host system.