Move comparison utilities to np_datetime;

[jbrockmendel]

we will need them available upstream of tslib

De-privatizes the appropriate names.

• closes #xxxx
• tests added / passed
• passes git diff upstream/master -u -- "*.py" | flake8 --diff
• whatsnew entry

[codecov]

Codecov Report

Merging #18080 into master will decrease coverage by 0.01%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #18080      +/-   ##
==========================================
- Coverage   91.25%   91.23%   -0.02%     
==========================================
  Files         163      163              
  Lines       50120    50120              
==========================================
- Hits        45737    45728       -9     
- Misses       4383     4392       +9

Flag	Coverage Δ
#multiple	89.04% <ø> (ø)	⬆️
#single	40.32% <ø> (-0.06%)	⬇️

Impacted Files	Coverage Δ
pandas/io/gbq.py	25% <0%> (-58.34%)	⬇️
pandas/core/frame.py	97.75% <0%> (-0.1%)	⬇️

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[jreback]

thanks!