PERF: (partial) fix for np_datetime.c performance regression
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Fixes (part) of the performance regression seen mentioned in #57951. The main performance improvements are the ~~ ~~refactoring of the "days" parsing logic~~ and the simpler calculation of ifrac.

The days parsing refactoring needed to be reverted. I overlooked an issue that happens for td close to int64 bounds there.
cc @WillAyd  This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this. Thanks for the pull request, but it appears to have gone stale. If interested in continuing, please merge in the main branch, address any review comments and/or failing tests, and we can reopen.