rustdoc: Refactor Impl.{synthetic,blanket_impl} into enum

This change has two advantages:

1. It makes the possible states clearer, and it makes it impossible to
   construct invalid states, such as a blanket impl that is also an auto
   trait impl.

2. It shrinks the size of `Impl` a bit, since now there is only one
   field, rather than two.

src/librustdoc/formats/cache.rs

@@ -228,7 +228,7 @@ impl<'a, 'tcx> DocFolder for CacheBuilder<'a, 'tcx> {
         // Collect all the implementors of traits.
         if let clean::ImplItem(ref i) = *item.kind {
             if let Some(trait_) = &i.trait_ {
-                if i.blanket_impl.is_none() {
+                if !i.is_blanket_impl() {
                     self.cache
                         .implementors
                         .entry(trait_.def_id())