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_ast_pretty/src/pprust/state.rs

@@ -1711,7 +1711,7 @@ impl<'a> State<'a> {
     fn print_expr_struct(
         &mut self,
         path: &ast::Path,
-        fields: &[ast::Field],
+        fields: &[ast::ExprField],
         rest: &ast::StructRest,
         attrs: &[ast::Attribute],
     ) {