Does't have a type for textual generation references

A GenRef type would take a string that is the textual representation of a ChunkId or an expression like "latest", and can convert that into a ChunkId.

We already do this, but using String, which is sub-optimal use of the type checker.

Later, we may want to support other textual expressions, such as "yesterday", or "one hour ago". We may also want to add support for "tagged" generations. None of this matters now, the GenRef type would be immediately useful.