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_hir/src/arena.rs

@@ -25,8 +25,8 @@ macro_rules! arena_types {
             [] generic_bound: rustc_hir::GenericBound<$tcx>,
             [] generic_param: rustc_hir::GenericParam<$tcx>,
             [] expr: rustc_hir::Expr<$tcx>,
-            [] field: rustc_hir::Field<$tcx>,
-            [] field_pat: rustc_hir::FieldPat<$tcx>,
+            [] expr_field: rustc_hir::ExprField<$tcx>,
+            [] pat_field: rustc_hir::PatField<$tcx>,
             [] fn_decl: rustc_hir::FnDecl<$tcx>,
             [] foreign_item: rustc_hir::ForeignItem<$tcx>,
             [few] foreign_item_ref: rustc_hir::ForeignItemRef<$tcx>,
@@ -42,7 +42,7 @@ macro_rules! arena_types {
             [] poly_trait_ref: rustc_hir::PolyTraitRef<$tcx>,
             [] qpath: rustc_hir::QPath<$tcx>,
             [] stmt: rustc_hir::Stmt<$tcx>,
-            [] struct_field: rustc_hir::StructField<$tcx>,
+            [] field_def: rustc_hir::FieldDef<$tcx>,
             [] trait_item_ref: rustc_hir::TraitItemRef,
             [] ty: rustc_hir::Ty<$tcx>,
             [] type_binding: rustc_hir::TypeBinding<$tcx>,