Implement better conversion to strings and better formatting using fmt/std::format

[KyleVaughn]

Currently, writing non-string arguments to C-strings or String uses snprintf to write to a buffer. This is not the most performant nor the most elegant solution to this problem. C++-20 adopted std::format (in particular std::format_to_n) for this purpose. We would like to use std::format, but the minimum compiler versions supported by UM2 do not implement std::format.

Since https://github.com/fmtlib/fmt is the reference implementation of std::format and is very widely used, it seems like a reasonable solution is to conditionally include this library. For compilers which support std::format, inclusion of the library can be omitted. In the future if the minimum UM2 standard is bumped to C++23 and we want to drop fmtlib entirely, the only required changes to the code would be the conditional aliasing of fmt vs std in a header.

• Replace usage of snprintf in String, the logger, and I/O with std::format_to_n
• Find an alternative to log.hpp's toBuffer. This might just effectively be std::format_to_n.
• Write a toBuffer equivalent for Vec.