Previously we only supported recursive types in structs. But there can
be other kinds of recursive types, like slices:
type RecursiveSlice []RecursiveSlice
This doesn't involve structs, so it led to infinite recursion in the
compiler. This fix avoids recursion at the proper level: at the place
where the named type is defined.
There were a few cases left where a named type would cause a crash in
the compiler. While going through enough code would have found them
eventually, I specifically looked for the `Type().(` pattern: a Type()
call that is then used in a type assert. Most of those were indeed bugs,
although for some I couldn't come up with a reproducer so I left them
as-is.
