feat(mediatypes): reimplement (and unvendor) mimeparse

[vytas7]

Unvendoring and rewriting mimeparse functionality:

• Fixes #864.
• Fixes #1367.
• Closes #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:

• [x] Remove debug print()s.
• [x] Create specialized exception classes, and use them instead ValueError.
• [x] Provide full code coverage.
• [x] 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[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

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

Additional details and impacted files

```diff @@ 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.