Add subsec_micros and subsec_millis methods to TimeDelta

[ggoetz]

This PR revives @botahamec 's PR #1351, which has been abandoned, and adds subsec_micros and subsec_millis to the public API for chrono::TimeDelta, for consistency with the API of std::time::Duration.

Summary of changes:

• expose TimeDelta constants like NANOS_PER_MICRO and companions in chrono's lib.rs. See the comment thread on the original PR description for the rationale behind the move.
• update the docstring for TimeDelta::subsec_nanos() and add a test for the method.
• add subsec_millis and subsec_micros, along with unit tests.

Fixes: #1348

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.07%. Comparing base (2b7a28e) to head (25936a4).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1668      +/-   ##
==========================================
+ Coverage   91.05%   91.07%   +0.01%     
==========================================
  Files          37       37              
  Lines       17402    17431      +29     
==========================================
+ Hits        15846    15875      +29     
  Misses       1556     1556              

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[djc]

Honestly I'm not convinced by the argument that these consts should be made public just so two of them can make a single appearance in a docstring. IMO it would be fine to leave the constants alone and use literals to explain the methods.

[ggoetz]

Honestly I'm not convinced by the argument that these consts should be made public just so two of them can make a single appearance in a docstring. IMO it would be fine to leave the constants alone and use literals to explain the methods.

Makes sense to me -- and there was precedent with the previous docstring for subsec_nanos to only use a literal without exposing the corresponding const. I've reverted the commit that exposed the constants.