Add 0.5.0 IDL and AST format

[mtdowling]

Issue #, if available:

Closes #189

Description of changes:

This commit adds support for version 0.5.0 of the AST. This makes loading and parsing the AST model much simpler and more approachable for other languages and tools like grep.

A TON of stuff has been changed in this PR. Lots of the changes were done using ad-hoc scripts I wrote to migrate from the old JSON AST to the new one (including scripts used to migrate the RST example). I looked at each RST example carefully, but they should still be double-checked. Test files were updated until they worked, so I'm confident that their right, but could use a very cursory glance at best.

Things to really look for:

• The JSON AST spec was updated significantly.
• Loading JSON models was updated. A single model loader is used for all JSON formats, and it then delegates to specific versions.
• We no longer have a bunch of logic for loading and parsing different versions of the IDL and comparing ranges of compatibility. We still support 0.4.0 and 0.5.0 and consider them compatible. Instead, we now just fail if we don't recognize the model version number. I think this is a much safer implementation choice.
• Traits that target shapes must also use absolute shape IDs. When using the IDL, this is no big deal since it support forward references. However, this will be an issue for JSON models that are migrated from 0.4.0 to 0.5.0. A warning is emitted when such an ID is found in idRef or references traits.
• smithyVersion was removed from Model. This really doesn't make sense because the version is relative to the version of smithy-model being used, not the version of the model files that were loaded. Put another way, you can't change the semantic model in memory based on the version loaded from a model file, so smithyVersion doesn't make sense.

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

[mtdowling]

This has been updated to include the AST, IDL, and protocol compliance tests. It's a rebase of those features and squashed into single commits for each. Nothing new here.