Copies changes from ion-11-encoding branch that are not specific to Ion 1.1

[popematt]

Issue #, if available:

None.

Description of changes:

Copies changes from ion-11-encoding branch back over to master.
All commits have links to the prior PRs that approved the changes.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 67.92%. Comparing base (3c1b6b1) to head (51b5f7d).
⚠️ Report is 120 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff              @@
##             master    #1096      +/-   ##
============================================
+ Coverage     67.23%   67.92%   +0.68%     
- Complexity     5484     5608     +124     
============================================
  Files           159      162       +3     
  Lines         23025    23281     +256     
  Branches       4126     4165      +39     
============================================
+ Hits          15481    15813     +332     
+ Misses         6262     6180      -82     
- Partials       1282     1288       +6     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[jobarr-amzn]

Thanks for pulling these languishing changes over!

[popematt]

As per offline discussion:

There are a number of other relevant commits but they had already been copied over, although not always with the same commit ID.

Commits with relevant content that were not copied include:

• 43518b5 because I think there's got to be a better solution, and I want to see if there is before we make this change.
• a0de643 because it's a very large change that affects Ion 1.0 and Ion 1.1 changes, so it cannot be cleanly cherry-picked. While this may be of some benefit, I want to wait and see if it's necessary (since we're trying a new reader architecture anyway) before manually re-creating these changes.