P0122R7 Span: bounds-safe views for sequences of objects

Editorial notes:
* Synopsis is now a numbered section.
* Removed "std::" from some type names in the class definition.
* Reordered Elements into canonical order.
* Reordered range expressions from "x >= 0 && x <= N" to the more mathematical form "0 <= x && x <= N".
* Removed extraneous parentheses from conditional expressions.
* Renamed pair-of-iterator parameters {first,last}Elem to just {first,last}.

Author: tkoeppe

source/compatibility.tex

@@ -1702,6 +1702,7 @@
 \effect
 The following \Cpp{} headers are new:
 \tcode{<any>},
+\tcode{<charconv>},
 \tcode{<execution>},
 \tcode{<filesystem>},
 \tcode{<memory_resource>},
@@ -1924,12 +1925,13 @@
 
 \rSec2[diff.cpp17.library]{\ref{library}: library introduction}
 
-\ref{headers}
+\diffref{headers}
 \change New headers.
 \rationale New functionality.
 \effect
 The following \Cpp{} headers are new:
 \tcode{<compare>},
+\tcode{<span>},
 \tcode{<syncstream>}, and
 \tcode{<version>}.
 Valid \CppXVII{} code that \tcode{\#include}{s} headers with these names may be