Replace post-increment and post-decrement boilerplate for iterators

Many iterators defined in the library repeat the same wording:

Effects: Equivalent to:

auto tmp = *this;
++*this;
return tmp;

We should add front matter which says that when no detailed specification is present for operator++(int) it has the semantics shown above, and the equivalent with -- for operator--(int).

This would probably belong in [functions.within.classes].

This touches a lot of the library and should probably get checked by LWG, but still seems editorial to me.

Start with [functions.within.classes], the location suggested by the issue, and inventory the repeated operator++(int) and operator--(int) specifications across the library. Confirm that the proposed front matter covers cases without detailed specifications, then verify the boilerplate can be replaced consistently and prepare the result for the indicated LWG review.