ast: Stop using Mod in Crate

Crate root is sufficiently different from `mod` items, at least at syntactic level.

Also remove customization point for "`mod` item or crate root" from AST visitors.

compiler/rustc_hir/src/hir.rs

@@ -615,7 +615,7 @@ pub struct WhereEqPredicate<'hir> {
     pub rhs_ty: &'hir Ty<'hir>,
 }
 
-#[derive(Encodable, Debug, HashStable_Generic)]
+#[derive(Default, Encodable, Debug, HashStable_Generic)]
 pub struct ModuleItems {
     // Use BTreeSets here so items are in the same order as in the
     // list of all items in Crate