replace traverse code with evaluation against metaschema

[karenetheridge]

As the work in JSMDO has shown, a schema's traverse phase is very similar to the act of evaluating the schema against its metaschema -- the work we do at traverse time to identify embedded identifiers etc can be replaced by similar code that is triggered by evaluation callbacks on the properties keyword.

One interesting counter-example is if a document changes its dialect (embedded $schema keyword) midway -- at that point we cannot keep evaluating the schema against the same metaschema, as now the metaschema has changed. (!)