Changes fmtId to return the FormatId type directly, and renames the
FormatId.render function to FormatId.format, so it can be used in a
format expression directly.
Why? Since `render` is private, you can't create functions that wrap
`fmtId` or `fmtIdFlags`, since you can't name the return type of those
functions outside of std itself.
The current setup _might_ be intentional? In which case I can live with
it, but I figured I'd make a small contrib to upstream zig :)
e12dc4947c