Clean pyomo collections module

[eminyouskn]

Fixes NA

Summary/Motivation:

This PR introduce some refactoring of the pyomo collections module.

Changes proposed in this PR:

• Create a separate file for the hasher.
• Remove OrderDict import from orderset.
• Sort imports.

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[jsiirola]

We can merge this, but it would be good to add a bit of documentation while we are refactoring.

[jsiirola]

We talked about this on the dev call, but I think it would be worth removing the _ -- Sphinx by default will not document private classes, attributes, etc.

Of course, we should also add documentation to this class...

[eminyouskn]

Thanks John for the review. I added a short documentation of the class. I let you tell me if it is clear enough.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 89.19%. Comparing base (7deb912) to head (1d92091).
⚠️ Report is 182 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #3708   +/-   ##
=======================================
  Coverage   89.19%   89.19%           
=======================================
  Files         892      893    +1     
  Lines      103100   103099    -1     
=======================================
  Hits        91956    91956           
+ Misses      11144    11143    -1     

Flag	Coverage Δ
builders	26.73% <65.15%> (+<0.01%)	⬆️
default	85.79% <100.00%> (?)
expensive	34.07% <80.30%> (?)
linux	86.95% <100.00%> (-1.97%)	⬇️
linux_other	86.95% <100.00%> (-0.01%)	⬇️
osx	83.29% <100.00%> (-0.01%)	⬇️
win	85.17% <100.00%> (+<0.01%)	⬆️
win_other	85.17% <100.00%> (+<0.01%)	⬆️

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[jsiirola]

Thanks! Some minor wording changes (the only real reason for the change is to line wrap, and to us the Sphinx "magic comment" syntax)