Add stringification for std::chrono::{duration,time_point}

Also hides std::chrono, std::pair and std::chrono::* behind
new configuration macros, CATCH_CONFIG_ENABLE_*_STRINGMAKER
to avoid dragging in <utility>, <tuple> and <chrono> in common
path, unless requested.

projects/SelfTest/ToStringPair.cpp

@@ -1,3 +1,4 @@
+#define CATCH_CONFIG_ENABLE_PAIR_STRINGMAKER
 #include "catch.hpp"
 
 TEST_CASE( "std::pair<int,std::string> -> toString", "[toString][pair]" ) {