Merge 2018-03 LWG Motion 15

Author: zygoloid

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