Improve throughput of TimeSpan.ToString/TryFormat with "g"/"G" (#19051)

Improve throughput of TimeSpan.ToString/TryFormat with "g"/"G" (#19051)

* Improve throughput of TimeSpan.ToString/TryFormat with "g"/"G"

TimeSpan has three standard formats: "c", "g", and "G".  Yesterday I updated its implementation with throughput improvements for "c" (the default) based on porting the design from Utf8Formatter; this PR does so for "g"/"G".

Initially I wasn't going to handle "g"/"G" as they factor in culture (Utf8Formatter doesn't), but even with accessing the current culture there are still significant wins to be had.  I was also going to keep the "c" and "g"/"G" implementations separate, to avoid bogging down the default "c" formatting with additional conditions needed to support "g"/"G", but the overhead incurred for that turns out to be minimal enough that it's worth keeping one implementation rather than two mostly-similar ones... the impact on "c" is mostly within noise.

This PR makes a significant throughput improvement for "g"/"G" formatting.  It also removes several unnecessary allocations, such that TryFormat with "g"/"G" is now allocation-free (and ToString just allocates the asked-for string).

* Address PR feedback