feat(mediatypes): reimplement (and unvendor) mimeparse

[vytas7]

Unvendoring and rewriting mimeparse functionality:

• Fixes Reverse the order in which Request.client_prefers matches media types #864.
• Fixes Prefer exact match without parameters to match with extra parameters. #1367.
• Closes Make it possible to unvendor python-mimeparse #2054.

I believe the code is more or less complete now, and I'm proposing it as is.

Some things that are, however, known to still be work in progress:

• Remove debug print()s.
• Create specialized exception classes, and use them instead ValueError.
• Provide full code coverage.
• Add all relevant news fragments.
• Tune the LRU cache decorators, esp on PyPy (where to use the "simple logic" passthrough). Not critical for release -- postponed to post-4.0.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (d45c06c) to head (532b44e).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff            @@
##            master     #2348   +/-   ##
=========================================
  Coverage   100.00%   100.00%           
=========================================
  Files           64        64           
  Lines         7624      7712   +88     
  Branches      1246      1267   +21     
=========================================
+ Hits          7624      7712   +88     

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[vytas7]

The code now has full unit test coverage, but I am still adding more cases.

[CaselIT]

Looks great. I've marked a few minor things