Note [Runtime representation of modules and tycons]

We generate a binding for M.$modName and M.$tcT for every module M and
data type T.  Things to think about

  - We want them to be economical on space; ideally pure data with no thunks.

  - We do this for every module (except this module GHC.Internal.Types), so we can't
    depend on anything else (eg string unpacking code)

That's why we have these terribly low-level representations.  The TrName
type lets us use the TrNameS constructor when allocating static data;
but we also need TrNameD for the case where we are deserialising a TyCon
or Module (for example when deserialising a TypeRep), in which case we
can't conveniently come up with an Addr#.