Issue title: Support UUIDs as schema references
 Issue body: In addition to schema references as strings and namespaced keywords, it would be very useful to refer to schemas via UUID.
 Comments: 
 Comment 0: @ikitommi I'm happy to open a PR for this, but I'm not quite sure where to start.  Could you point me to the places where refs are currently resolved/substituted for strings, namespaced keywords, etc.?
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