Replace post-increment and post-decrement boilerplate for iterators

[jwakely]

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.