This is needed to be able to round trip enums.
If there is no codegen for an enum that is read from the datastore we
now return a types.Enum which can be serialized back to the same
sequence.
The TypeRef function for a Noms Struct should be the (Name, PkgRef) and
not the description of the struct fields. This is important because
when serializing we need to write the package ref.
Towards #281#304
These were two representations of, essentially, the same information.
They were separate because they provided different APIs to similar
information, but the APIs became more similar once we started using
native types (as opposed to Noms types) for the various Make*TypeRef()
functions.
Unifying these is a big step to unifying parse.Package and types.Package,
which is pretty necessary for dealing with imported packages.
Fixes issue #338
We want to explore encoding type information about Noms data in
the Noms database. So, we need some way to describe types. This
takes the shortest path to making a Noms type called "TypeRef" that
is a peer of Set, Map et al and can describe all the types we currently
use.
