Begin migrating from fmt to c++20 format
libstdc++ and libc++ on all of our supported distros is now new enough to support <format>.

This should be mostly straightforward to implement, the only weird one will be fextl::fmt::vformat since it uses the fmt::basic_memory_buffer for custom allocator support. There's no direct 1:1 replacement in the C++ standard so will need some massaging.

This will remove one more external project dependency in a relatively straight forward way. Low-prio but shouldn't be terrible to do.