Track the arguments and witnesses on values indirectly. (#1335)

This avoids the need to make copies of potentially large maps when
the same bindings are used in multiple values, such as when forming
a bound method value.

explorer/ast/bindings.cpp

@@ -0,0 +1,14 @@
+// Part of the Carbon Language project, under the Apache License v2.0 with LLVM
+// Exceptions. See /LICENSE for license information.
+// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
+
+#include "explorer/ast/bindings.h"
+
+namespace Carbon {
+
+auto Bindings::None() -> Nonnull<const Bindings*> {
+  static Nonnull<const Bindings*> bindings = new Bindings({}, {});
+  return bindings;
+}
+
+}  // namespace Carbon