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/value.cpp

@@ -97,7 +97,7 @@ struct NestedValueVisitor {
     // which is not "within" this value, so we shouldn't visit it.
     return true;
   }
-  auto Visit(const VTable&) -> bool { return true; }
+  auto Visit(const VTable*) -> bool { return true; }
 
   llvm::function_ref<bool(const Value*)> callback;
 };