Use TryFrom for TimeSpec/Duration conversions which may underflow/overflow

[sirhcel]

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What does this PR do

• This is a follow-up to #2479 for switching from From to TryFrom for conversions between TimeSpec and Duration which may underflow or overflow
• I learned that From and TryFrom can not co-exist and so adding the latter and deprecating the former does not seem feasible and it looks like we are left with a breaking change here
• I'm starting this as a draft PR for
  • Discussing the desired level of detail for reporting errors from TryFrom. The underlying integer conversions return a TryFromIntError which informs that the conversion has failed but gives no specific reason.
  • Getting feedback from CI
  • Adding tests for the conversions once the course for this PR is set
  • Adding a change log once the course for this PR is set

Checklist:

• [x] I have read CONTRIBUTING.md
• [ ] I have written necessary tests and rustdoc comments
• [ ] A change log has been added if this PR modifies nix's API