(GH-538) Return Schema for subschema extension methods

This change:

- Updates the following functions to return instances of `schemars::Schema`
  instead of `Map<String, Value>`, since the returned data is _always_ a
  subschema, if it exists:

  - `get_defs_subschema_from_id()`
  - `get_defs_subschema_from_id_mut()`
  - `get_defs_subschema_from_reference()`
  - `get_defs_subschema_from_reference_mut()`
  - `get_property_subschema()`
  - `get_property_subschema_mut()`
- Removes the type aliases `Object` (for `Map<String, Value>`) and `Array`
  (for `Vec<Value>`), as these conveniences weren't saving much typing and
  Rust Analyzer wasn't always plumbing them through for IntelliSense. The
  uses of these aliases now revert to calling the underlying types.
- Updates documentation and tests for the modified functions.

Author: michaeltlombardi