Implement canonicalization of Value and Element (#3024)

See arena.h for discussion of what canonicalization means in this
context. This is primarily intended to support implementing a memo table
of template instantiations to resolve #2951, but could be useful for
other purposes as well.

Additional changes:
- Pass `VTable` constructor parameters by pointer, to avoid the need to
define `operator==` and `hash_value` for it.
- Clean up the recurring pattern of allocating identical `NamedElement`s
on the stack and heap. Instead we always allocate it on the heap and
pass it by pointer.
- Add `Print()` and `Dump()` to `Bindings` as a debugging convenience.

---------

Co-authored-by: Jon Ross-Perkins <jperkins@google.com>
Co-authored-by: Richard Smith <richard@metafoo.co.uk>

explorer/common/decompose_test.cpp

@@ -0,0 +1,39 @@
+// 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/common/decompose.h"
+
+#include <gtest/gtest.h>
+
+namespace Carbon {
+namespace {
+
+struct Decomposeable : public HashFromDecompose<Decomposeable> {
+  template <typename F>
+  auto Decompose(F f) const {
+    return f(i, s);
+  }
+
+  int i = 0;
+  std::string s;
+};
+
+TEST(HashFromDecomposeTest, EqualValues) {
+  Decomposeable d1 = {.i = 42, .s = "foo"};
+  Decomposeable d2 = {.i = 42, .s = "foo"};
+
+  EXPECT_TRUE(d1 == d2);
+  EXPECT_TRUE(hash_value(d1) == hash_value(d2));
+}
+
+TEST(HashFromDecomposeTest, NonEqualValues) {
+  Decomposeable d1 = {.i = 42, .s = "foo"};
+  Decomposeable d2 = {.i = 42, .s = "bar"};
+
+  EXPECT_FALSE(d1 == d2);
+  EXPECT_FALSE(hash_value(d1) == hash_value(d2));
+}
+
+}  // namespace
+}  // namespace Carbon