Add <stdbit.h> bit manipulations

This replaces (in C code) the custom bit manipulation functions with the much broader and more systematic standard set. The main point is to afford better optimisations by capable compilers. In some cases, this does simplify the existing code though.

For older compilers, the same GCC+LLVM optimisations are supported as before. Adding new optimisations for other legacy compilers is possible but of no obvious use and out of scope.

Meson is left out since it is not working properly yet.