Allow YANG parser to not create SchemaPaths in SchemaNodes

Add an option (environment-default) where YANG parser will not expose SchemaNode.getPath(), but rather will throw UnsupportedOperationException when the method is called.

Class layout should not suffer much – the field currently used for SchemaPath can be made polymorphic (i.e. declared as Object) and dispatch getQName() and getPath() based on either run-time type of the object or based on a runtime constant.