Add managed writer for Ion 1.1 with basic round-trip testing

[popematt]

Issue #, if available: None

Description of changes:

This adds a "managed" writer for Ion 1.1 that is generic over whether the raw encoding is Text or Binary.

Caveats:

• There are a lot of classes that need unit testing
• I had to change the visibility of at least a half dozen existing classes. We will need to revisit the package organization and the visibility of those classes.
• There is a lot of documentation that is not complete yet.
• Does not support shared symbol tables.
• ...probably others? I've exhausted my mental capacity for the week, and I just need to get this out.

However, this does have some basic round-trip tests using the data in ion-tests and this is enough that we can start building other things around it an incrementally filling in the gaps.

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