Add a function to expose the ISL model for Schema and Type

[popematt]

It should be possible to use the Schema System to load one (or more) schemas, and then get ISL model for the schema or for a type in the schema.

The ISL model should be exposed so that users can inspect and possibly manipulate the types and schemas using a typed data model (as opposed to working with straight Ion).

Any open content should also be exposed so that users can inspect any open content that might be present in the schema or type, but that is dependent on #6.