Version the metamodel and catch drift before it corrupts the graph

[jimador]

Changes to the metamodel are silent. There's no way to see what changed between versions, and
mention-types that drift away from the model quietly land in the graph and corrupt it.

Version the metamodel so each version can be diffed against the last, and quarantine drifting types
for review instead of letting them silently through.

Use cases

• See exactly what changed between two metamodel versions before adopting the new one.
• Catch a mention-type that has drifted from the model and quarantine it for review instead of
  letting it corrupt the graph.
• Roll the metamodel forward deliberately, with a record of each version to look back on.