Merge 2019-07 LWG Motion 19

P1650R0 Output std::chrono::days with 'd' suffix

Fixes #3023.

Author: zygoloid

source/time.tex

@@ -1992,6 +1992,7 @@
 \item Otherwise, if \tcode{Period::type} is \tcode{exa}, the suffix is \tcode{"Es"}.
 \item Otherwise, if \tcode{Period::type} is \tcode{ratio<60>}, the suffix is \tcode{"min"}.
 \item Otherwise, if \tcode{Period::type} is \tcode{ratio<3600>}, the suffix is \tcode{"h"}.
+\item Otherwise, if \tcode{Period::type} is \tcode{ratio<86400>}, the suffix is \tcode{"d"}.
 \item Otherwise, if \tcode{Period::type::den == 1}, the suffix is \tcode{"[\placeholder{num}]s"}.
 \item Otherwise, the suffix is \tcode{"[\placeholder{num}/\placeholder{den}]s"}.
 \end{itemize}