Introduce util::format

[mazunki]

As requested in #2309 (comment), byte-formatters now reside in a standalone namespace.

I have also introduced a stringifier for errno values. The standard has strerror() for the corresponding message, but not the name. This provides errno_name(errno) for this purpose.

There's also a convenience helper for things that don't support string-views: errno_cstr(errno). This is preferred over explicitly doing errno_name(errno).data() in corresponding locations because this way we can eventually deprecate c-strings from a single location without having to track down who uses printf-style specifiers.

Sadly, there is no convenient way to put these into an enum class and have the compiler turn it into a string. There are hopes for this in C++26, see: https://www.open-std.org/jtc1/sc22/wg21/docs/papers/2025/p2996r13.html

[mazunki]

Actually, we have api/utils/units.hpp, which I had forgot someone mentioned. While Byte_r is alright, it doesn't provide std::formatter<Byte_r> which is desired for std::format().

There are currently two different ways to represent bytes, this PR doesn't change that; none of which are friendly to formatting.

For what it's worth, we might want to introduce Errno(n) for the same purpose, so we can have std::formatter<Errno> too.