Finally, remove the parent type metadata argument from type
constructors.
Now that type constructors don't take a parent metadata pointer,
we can hit some asserts concerning type constructors that do not
have any parameters. This happens when you define a concrete type
in a fully-constrained extension of a generic type.
A more efficient ABI would use concrete type metadata for these
cases, but that would be a bigger change that we can do later, so
for now just relax these assertions.
This resolves a runtime crasher since a circular metadata case is
no longer circular. I renamed the crasher to reference the more
specific radar since the more general issue of circular metadata
is still unresolved.
"minimal" is defined as the set of requirements that would be
passed to a function with the type's generic signature that
takes the thick metadata of the parent type as its only argument.
