Upgrade to bitflags 2.0

[juntyr]

Supersedes #442. Updates the bitflags dependency to 2.0

Breaking change:

• bitflags 2.0 serializes differently, now using the names instead of (bits: 42). Did we ever commit to how Extensions is serialized?

@kvark bitflags-serial still uses bitflags 1.0, so I've removed the related test for now. Feel free to re-add it or ping me once bitflags-serial is updated

• I've included my change in CHANGELOG.md

[juntyr]

?r @torkleyy

[codecov-commenter]

Codecov Report

❗ No coverage uploaded for pull request base (master@c0d7421). Click here to learn what that means.
Patch coverage: 83.33% of modified lines in pull request are covered.

❗ Current head ff48634 differs from pull request most recent head 951c88b. Consider uploading reports for the commit 951c88b to get more accurate results

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Additional details and impacted files

@@            Coverage Diff            @@
##             master     #443   +/-   ##
=========================================
  Coverage          ?   86.50%           
=========================================
  Files             ?       61           
  Lines             ?     7983           
  Branches          ?        0           
=========================================
  Hits              ?     6906           
  Misses            ?     1077           
  Partials          ?        0           

Impacted Files	Coverage Δ
src/de/mod.rs	75.13% <ø> (ø)
src/extensions.rs	81.57% <50.00%> (ø)
tests/152_bitflags.rs	52.05% <85.71%> (ø)
src/value/raw.rs	88.65% <100.00%> (ø)

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