make polymorphic_allocator throw consistent type (LWG 3237)
Add __cpp_lib_unwrap_ref feature test macro
midpoint should not constrain T is complete (LWG 3200)
span's deduction-guide for built-in arrays doesn't work (LWG 3369)
Remove std::span::cbegin and std::span::cend (LWG 3320)

Tested powerpc64le-linux, committed to master.