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/ast/bindings.cpp

@@ -8,6 +8,7 @@
 #include "explorer/ast/impl_binding.h"
 #include "explorer/ast/pattern.h"
 #include "explorer/ast/value.h"
+#include "llvm/ADT/StringExtras.h"
 
 namespace Carbon {
 
@@ -41,6 +42,18 @@ auto Bindings::None() -> Nonnull<const Bindings*> {
   return bindings;
 }
 
+void Bindings::Print(llvm::raw_ostream& out) const {
+  out << "{";
+  llvm::ListSeparator sep;
+  for (const auto& [name, value] : args_) {
+    out << sep << *name << " -> " << *value;
+  }
+  for (const auto& [name, value] : witnesses_) {
+    out << sep << *name << " -> " << *value;
+  }
+  out << "}";
+}
+
 auto Bindings::SymbolicIdentity(
     Nonnull<Arena*> arena,
     llvm::ArrayRef<Nonnull<const GenericBinding*>> bindings)