Make use of C++20 bit manipulation features

[axic]

We are currently using compiler builtins:

• __builtin_clz
• __builtin_clzll
• __builtin_ctz
• __builtin_ctzll
• __builtin_popcount
• __builtin_popcountll

These can be replaced with the new C++20 features proposed (see https://en.cppreference.com/w/cpp/numeric).

For an idea how to write such a wrapper, check out lib/fizzy/cxx20/span.hpp.

[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