Integer utilities

[eisenwave]

We already have P3605 for isqrt and P3724 for integer division, but there are many more integer utilities that could be provided, such as:

• std::ipow(base, exp) for integer expontentiation
• std::ilog(base, x) for integer logarithms
• much faster versions of the above that accept a constant base
• std::uabs (added to C2y, could be ported over pre-emptively or during C2y rebase in a few years)
• std::max_mag and std::min_mag for picking the value with greater or lower magnitude
• modular exponentionation, modular logarithm, and other modular arithmetic utilities
• wrapping signed arithmetic operations without UB, like std::add_wrap(x, y)