Big assertion capture refactoring.

- moved as much logic out of the macros as possible
- moved most logic into new ResultBuilder class, which wraps ExpressionResultBuilder (may take it over next), subsumes ResultAction and also takes place of ExpressionDecomposer.

This introduces many SRP violations - but all in the name of minimising macro logic!

projects/SelfTest/MessageTests.cpp

@@ -7,6 +7,7 @@
  */
 
 #include "catch.hpp"
+#include <iostream>
 
 #ifdef __clang__
 #pragma clang diagnostic ignored "-Wc++98-compat-pedantic"