common_iterator::operator-> is underconstrained

[CaseyCarter]

As specified in #318. The "proxy case" constructs an object of type value_type_t<I> from reference_t<I> but does not require Constructible<value_type_t<I>, reference_t<I>>().

Proposed Resolution

Change the class synopsis of common_iterator ([common.iterator]) as follows (includes part of the resolution of #368):

 see below operator*();
 see below operator*() const requires dereferenceable<I const>;
-see below operator->() const requires Readable<I const>();
+see below operator->() const requires see below;

To [common.iter.op.ref], add a p3 that reads:

3. The expression in the requires clause is equivalent to:

Readable<I const>() &&
  (requires (const I& i) { i.operator->(); } ||
   is_reference<reference_t<I>>::value ||
   Constructible<value_type_t<I>, reference_t<I>>())

[ericniebler]

@CaseyCarter for your review