Otherwise we can get in trouble when a local type is named, say,
'Sequence'.
Also contains test updates and a fix for Harlan's previous commit,
which actually affects all typealiases, not just those in the Builtin
module.
The recovery logic was erronously kicking in, because it was comparing
the substituted underlying type with the declaration's underlying type.
For a generic typealias, these never equal, so instead, serialize the
unsubstituted type, and substitute it in deserialization.
