Thank you for the review!

- I moved the concepts back into the class and changed them to
template aliases because class local concepts aren't a thing that
works.
     + They are not inline constexpr bools because that ICE's GCC
pretty hard, as you explained to me earlier, so we use ::value in some
places rather than _v
- I named the alias __is_array_convertible, to match what the intent
is, and named the template parameters very explicitly.
     + I moved it into type_traits, but I didn't change
bits/unique_ptr.h or shared_ptr yet because I'm not confident I can do
that quickly in one go.
     + (And it should probably be a different patch anyways, but it
should be in the right place when we introduce it here.)

---------

2019-11-21  JeanHeyd "ThePhD" Meneide  <phdofthehouse@gmail.com>

        * include/std/span: Use concepts and ranges:: calls
          instead of enable_if and friends.
        * include/std/ranges: tear basic concepts out and put in
          range_access.h for "ranges lite" internal header.
        * include/std/type_traits: __is_array_convertible trait. Is common
          to unique/shared_ptr as well (preparation for changing
          unique/shared_ptr soon).
       * include/bits/range_access.h: Remove __adl_* versions
          of access functions.