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.

CMakeLists.txt

@@ -68,6 +68,7 @@ set(TEST_SOURCES
         ${SELF_TEST_DIR}/PartTrackerTests.cpp
         ${SELF_TEST_DIR}/TagAliasTests.cpp
         ${SELF_TEST_DIR}/TestMain.cpp
+        ${SELF_TEST_DIR}/ToStringChrono.cpp
         ${SELF_TEST_DIR}/ToStringGeneralTests.cpp
         ${SELF_TEST_DIR}/ToStringPair.cpp
         ${SELF_TEST_DIR}/ToStringTuple.cpp