1. Use `fmt::format_int` directly instead of parsing a format string.
2. Use `AppendStrView`.
3. Define the varargs versions using fold expressions when available.
4. Add tests.
Adds simple string / integer concatenation functions.
Many of the uses of `fmt::format` are simply concatenation
of a few strings and integers.
`StrCat` is an easier-to-read alternative to such uses of `fmt`.
Gleb Mazovetskiy
obligaron