SI-6187 Make partial functions re-typable

 - `New(tpe)` doesn't survive a `resetAttrs` / typecheck; use
   a name instead.
 - 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. Apart from making the tree re-typable, it
   also exposes the true code structure to macros, which
   is important if they need to perform other code
   transformations.
 - Install original trees on the declared types of the
   parameters of the `applyOrElse` method to ensure that
   references to them within the method pick up the correct
   type parameter skolems upon retypechecking.
 - Propagate `TypeTree#original` through `copyAttrs`,
   which is called during tree duplication / `TreeCopiers`.
   Without this, the original trees that we installed were
   not visible anymore during `ResetAttrs`.

We are not able to reify partial functions yet -- the
particular sticking point is reification of the parentage
which is only available in the `ClassInfoType`.

Review by @adriaanm / @xeno-by 

I'm using `typedCase` now, based on your prior review. The `asInstanceOf` is also gone.