formatted_size with format specifiers & FMT_COMPILE

[elbeno]

formatted_size doesn't work with format specifications when using FMT_COMPILE.
(See also: issue #2141)

https://godbolt.org/z/G595Y78ns

CE currently uses 10.0.0 but the same issue is on master.
It does work with {:d} but not with {:x} or {:b}. {:s} doesn't work either (obviously for a stringish case). I haven't tried more complex format specs. {:x} and {:b} work if I mark https://github.com/fmtlib/fmt/blob/master/include/fmt/format.h#L1390 as constexpr, but I'm not sure that's a good fix (and doesn't address {:s}).

[vitaut]

Making format_uint (conditionally) constexpr is the right solution and a PR is welcome. Similarly, for string formatting some additional constexprification is needed.