WIP SI-6187 Make partial function re-typing friendly

 - New(tpe) doesn't survive a resetAttrs / typecheck; use
   a name instead.
 - Copy original trees in TypeTree#copyAttrs. This fix is
   analogous to the fix that copied the `wasEmpty` field.
 - Abandon the tree attachment that passed the default
   case from `typer` to `patmat`; this tree eluded the
   attribute reset performed in the macro. Instead, add it
   to the match.

   TODO: suppress the unreachability warning that should
   show up given the fix in b49eaef.