Enforce reference scoping rules within lineages

[sdboyer]

There are a number of lineage validity rules we want to introduce about what references are allowed to be made within different parts of lineage definitions. An example we've known for a while is that we may want to disallow references to any types defined outside of the schema from within a schema, as it introduces the possibility of subtly breaking thema's guarantees when the referenced type gets changed.

This issue is intended as a running list of such rules. Membership on the list does not mean the rule is finally decided - more of a proposal.

• #Lens children MUST NOT contain references to anything outside the scope of the lens itself
• #Lens.result children MUST NOT contain references to the lacunas section in that lens. (#Lens.lacunas children MAY contain references to the result section.)