Modify JSON schema $ref resolution strategy to support resolving values internal to the schema

[mattkindy]

Overview

Previously, only $ref that were directed to #/$defs could be resolved via the use of a pre-loaded definitions variable. This PR modifies the behaviour of $ref resolutions by instead using a resolver function, which is generated to scope to a particular schema or subschema.

• This is still preloaded with #/$defs as before
• $ref keys are split to traverse through the scoped schema. When found, they are cached for re-use to avoid re-traversing
• In expand_json_schema, $id is an identifier of a scope change, and a new resolver linked to the schema for that scope is used for its children (e.g. for supporting bundling)
• Recursion is supported -- note that no effort is made to identify if this recursion is valid or could lead to infinite loops

Fixes #310 for local references (i.e. doesn't attempt to address remote schemas)

[brandonwillard]

This is great, and much appreciated, @mattkindy!