make common_iterator<I, S> require copyable<I> (LWG 3385)
Add default-initializers to views (LWG 3364)
Simplify std::three_way_comparable_with (LWG 3360)
Simplify std::totally_ordered (LWG 3331)
Simplify std::totally_ordered_with (LWG 3329)

Tested powerpc64le-linux, committed to master.