Allow CIF invariants to be named

We discussed as part of #319 (closed) how it can be useful to have named invariants. This allows them to be referred to, for instance to 'select' invariants. It would also allow for traceability to requirement documents.

We could just allow an optional <identifier>: prefix for invariants in the textual syntax, and add a String-typed name field to the Invariant class.

Obviously, as for any language change/extension, we would need to carefully consider all implications.