[C++11] Sink the iterator over a Value's users into the Value type
itself and teach it to convert between the non-const and const variants.
De-templatetize its usage in APIs to just use the const variant which
always works for those use cases. Also, rename its implementation to
reflect that it is an iterator over *users* not over *uses*.

This is a step toward providing both iterator and range support for
walking the *uses* distinct from the *users*. In a subsequent patch this
will get renamed to make it clear that this is an adaptor over the
fundamental use iterator.