Adds TypeRefWalker to support type reference resolution

[popematt]

Issue #, if available:

#228

Description of changes:

• Creates a resolver module
• Moves SchemaStore to the resolver module
• Moves/refactors HasTypeReferences into TypeRefVisitor and TypeRefWalker
• Add TypeRefWalker implementations for the whole model
• Updates a few builder methods that had ugly signatures—e.g. they were using a specific type Something instead of T: Into<Something>.
• Minor fix in violation_recorder.rs to get cargo doc to stop complaining about unclosed HTML tags.

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