ast/hir: Rename field-related structures

StructField -> FieldDef ("field definition")
Field -> ExprField ("expression field", not "field expression")
FieldPat -> PatField ("pattern field", not "field pattern")

Also rename visiting and other methods working on them.

compiler/rustc_privacy/src/lib.rs

@@ -1698,9 +1698,9 @@ impl<'a, 'tcx> Visitor<'tcx> for ObsoleteVisiblePrivateTypesVisitor<'a, 'tcx> {
         }
     }
 
-    fn visit_struct_field(&mut self, s: &'tcx hir::StructField<'tcx>) {
+    fn visit_field_def(&mut self, s: &'tcx hir::FieldDef<'tcx>) {
         if s.vis.node.is_pub() || self.in_variant {
-            intravisit::walk_struct_field(self, s);
+            intravisit::walk_field_def(self, s);
         }
     }