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!

include/internal/catch_interfaces_runner.h

@@ -13,6 +13,7 @@ namespace Catch {
 
     struct IRunner {
         virtual ~IRunner();
+        virtual bool aborting() const = 0;
     };
 }