Month and weekday arithmetic methods

[HubertK05]

This PR includes:

• nth_prev() method for Weekday,
• nth_prev() and nth_next() methods for Month,
• tests for methods listed above.

Closes #593.

[codecov[bot]]

Codecov Report

Merging #596 (af0a076) into main (52e5cee) will increase coverage by 0.0%. The diff coverage is 100.0%.

@@          Coverage Diff          @@
##            main    #596   +/-   ##
=====================================
  Coverage   95.7%   95.8%           
=====================================
  Files         79      79           
  Lines       8818    8867   +49     
=====================================
+ Hits        8443    8492   +49     
  Misses       375     375           

Impacted Files	Coverage Δ
time/src/month.rs	100.0% <100.0%> (ø)
time/src/weekday.rs	100.0% <100.0%> (ø)

... and 1 file with indirect coverage changes

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[jhpratt]

Looks great. Thank you!