adds changes for Ord implementation of Timestamp

[desaikd]

Description of changes:
This PR works on changes for Ord implementation of Timestamp to consider both datetime and fractional seconds for comparison.

• considers datetime comparison result as well as fractional seconds
  comparison result for Ord
• changes fractional_seconds_equal() to convert Option<&Mantissa> to &Mantissa, where default value Mantissa::Digits(0) is used for None. This guarantees a fractional_seconds value to be present at all times and makes it easier to compare those fractional seconds.

Test:

• adds test case for fractional second and year precision timestamps
  ordering

[codecov]

Codecov Report

Merging #410 (5bff7cc) into main (9445115) will decrease coverage by 0.00%.
The diff coverage is 83.78%.

@@            Coverage Diff             @@
##             main     #410      +/-   ##
==========================================
- Coverage   88.47%   88.47%   -0.01%     
==========================================
  Files          82       82              
  Lines       14059    14076      +17     
==========================================
+ Hits        12439    12454      +15     
- Misses       1620     1622       +2     

Impacted Files	Coverage Δ
src/types/timestamp.rs	87.81% <83.78%> (+0.10%)	⬆️
src/text/raw_text_reader.rs	90.42% <0.00%> (-0.17%)	⬇️

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[zslayton]

🚀