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_middle/src/hir/map/collector.rs

@@ -554,10 +554,10 @@ impl<'a, 'hir> Visitor<'hir> for NodeCollector<'a, 'hir> {
         });
     }
 
-    fn visit_struct_field(&mut self, field: &'hir StructField<'hir>) {
+    fn visit_field_def(&mut self, field: &'hir FieldDef<'hir>) {
         self.insert(field.span, field.hir_id, Node::Field(field));
         self.with_parent(field.hir_id, |this| {
-            intravisit::walk_struct_field(this, field);
+            intravisit::walk_field_def(this, field);
         });
     }