COMP: Improve const correctness for ValueIterators

[hjmjohnson]

The protected deref method had inconsistent interface
of being a const function that returned a non-const
reference. Resolves #914.

[cdunn2001]

Technically this broke binary compatibility, so this should have been a major version bump.

I guess we get away with this because probably nobody uses this method. If nobody complains, we can keep it.

[BillyDonahue]

I have to argue with the original premise of this change.

I did make the argument in the thread for issue #914 but received no response.

#914 (comment)

Iterators should model pointer. All standard container itererators do as well.

All 4 of these possiblities exist:

    T*
    const T*
    T*const
    const T*const

The constness of the pointer is independent of the constness of the thing to which it points.
Same with iterators. deref() should continue to be const method of iterator.

The right signatures are:

Value& ValueIterator::deref() const;
const Value& ConstValueIterator::deref() const;