Submdspan lwg feedback 23 06 13 (#388)

* Introduce index-pair-like

* Remove old commented text

* Fix index-pair-like to use correct convertibility

* Make submdspan_mapping-impl nto take a src argument

* Some more stuff

* More indetified fixes

* Other stuff

* de-ice

* ci test

* Testing the test

* Let de-ice be constexpr

* Forgot a couple de-ice and replace satisfy -> model integral-constant-like

* Fixup first_ arguments

* If [the] above conditions

* [the expression] blah is well[-]formed

* Add mandates is a specialization of submdspan_mapping_result

* Typo Extens -> Extents

* is_convertible_v<bla, index_type> -> convertible_to<bla, index_type> is modelled

* Update submdspan html

* well-formed when treated as an unevaluated operand

* $S_k$ models convertible_to<index_type>

* Prefer typename -> class and $S_k$ models index-pair-like

* specialization of strided_slice, then bla

* remove_reference_t<decltype(src)>::bla

* submdspan-mapping-impl add // exposition only comment

* function[']s template

* *all* -> all

* Rephrase note in submdspan_mapping

* math fonts and other stuff

* Extents{. -> ::}rank

* a -> grid3d

* stuff

* I think we did it

* Renumbering

* Update html

* is_convertible_v<bla> [is true]

* Update submdspan html

* Remove , or

* Update submdspan html

* More stuff from LWG

* Update submdspan html

* Update revision number

* Update submdspan html

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Co-authored-by: Damien L-G <[email protected]>
Co-authored-by: LOFT <[email protected]>