Introduce compile time toggle to remove Matchers from TU

The toggle is `CATCH_CONFIG_DISABLE_MATCHERS` and the only use is
to speed up compilation of small TUs. For large ones it is likely
insignificant, because the speed up is constant relative to
number of tests/assertions in TU.

include/internal/catch_matchers_string.h

@@ -8,6 +8,8 @@
 #ifndef TWOBLUECUBES_CATCH_MATCHERS_STRING_H_INCLUDED
 #define TWOBLUECUBES_CATCH_MATCHERS_STRING_H_INCLUDED
 
+#if defined(CATCH_CONFIG_DISABLE_MATCHERS)
+
 #include "catch_matchers.hpp"
 
 #include <string>
@@ -66,4 +68,6 @@ namespace Matchers {
 } // namespace Matchers
 } // namespace Catch
 
+#endif // CATCH_CONFIG_DISABLE_MATCHERS
+
 #endif // TWOBLUECUBES_CATCH_MATCHERS_STRING_H_INCLUDED