adds implementation for `document` type

[desaikd]

Issue #103

Description of changes: This PR adds implementation of document type.

Ion Schema specification: https://amzn.github.io/ion-schema/docs/spec.html#type-system

document: represents a series of top-level values

List of changes:

• adds IonSchemaElement implementation
• TypeRef validate method now uses IonSchemaElement as input
• TypeDefinitionImpl's parse_from_isl_type method now returns a TypeId instead of TypeDefinitionImpl
• adds nullable annotation check for non built in type references to return an error
• Remove document type tests from skip list
• adds id() for TypeRef to get the TypeId of that TypeRef
• adds doc examples

Tests: adds unit tests for document type (removes previously unsupported tests from skip list)

[desaikd]

Suggested review changes:

• Removes logic to convert document type to an SExpression
• adds changes to use IonSchemaElement for validation
• changed all constraint and type validation logic to use IonSchemaElement as input
• Added a Display implementation of IonSchemaElement to use it for violation messages
• added changes for derived types that are preloaded into TypeStore
• modified tests according to above changes
• Renamed IonSchemaElement enum variant

[popematt]

Overall, this approach looks really good. Just a few minor questions/comments.