Make use of C++20 bit manipulation features about fizzy

[herobank110]

Hello, I have been reading about c++20 features and would like to try solve this issue.

[chfast]

Hi @herobank110,

you can start with a simple task as follows:

• move the implementation of popcount (https://github.com/wasmx/fizzy/blob/master/lib/fizzy/execute.cpp#L328-L331) to cxx20/bit.hpp
• make it's function signature exactly as in C++20: https://en.cppreference.com/w/cpp/numeric/popcount.

[chfast]

Correction, start with something other than popcount as it is being done in #551.

[herobank110]

I made an wrapper of countl_zero. Please can you check it? #570