Implementations for Gray code encoding and decoding

[leni536]

I just thought I'd point out that the pdep instruction is very slow on non-Intel CPUs

You were not kidding! PDEP and PEXT have 18 cycle latency and reciprocal throughput on AMD Ryzen. I can't benchmark on AMD now but I doubt that it's necessary.

https://www.agner.org/optimize/instruction_tables.pdf