Meta: Ranges and iterator unwrapping

[CaseyCarter]

We've identified several issues with the iterator checking/unwrapping machinery, all of which should probably have fixes designed at the same time to avoid conflict:

• <algorithm>: Unwrapping output iterators in range algorithms #893 "<algorithm>: Unwrapping output iterators in range algorithms"
• <algorithm>: unqualified calls to _Adl_verify_range incorrectly cause instantiation #1596 "<algorithm>: unqualified calls to _Adl_verify_range incorrectly cause instantiation"
• <algorithm>: Re-evaluate iterator unwrapping design #1760 "<algorithm>: Re-evaluate iterator unwrapping design"
• <algorithm>: ranges::find() rejects some sentinels due to iterator unwrapping #2591 "<algorithm>: ranges::find rejects some sentinels due to iterator unwrapping" (A much more widespread issue than the title suggests)
• <memory>: ranges::uninitialized_move_n and ranges::uninitialized_copy_n missing move to rewrap input_iterator #2962 "<memory>: ranges::uninitialized_move_n and ranges::uninitialized_copy_n missing move to rewrap input_iterator (Every bit as narrow an issue as the title suggests)
• _Get_unwrapped et. al. should (almost) always be noexcept #2989 "<xutility>: _Get_unwrapped et al. should always be noexcept"
• Many view iterators do not contain an _Unwrapped() member function #2997 Many view iterators do not contain an `_Unwrapped() member function
• <iterator>: move_sentinel could be unwrappable #3009 "<iterator>: move_sentinel should be unwrappable"
• Range algorithms don't work when used with unwrappable iterators and custom sentinels #3010 "Range algorithms don't work when used with unwrappable iterators and custom sentinels"
• STL: Unwrapping class / pointer pairs #3050 "STL: Unwrapping class / pointer pairs"