Split out LazyExpr into its own header

This enables us to avoid `catch_reporter_bases.hpp` being indirectly
dependent on `catch_tostring.hpp`, cutting apart quite a bit indirect
inclusions.

src/catch2/catch_assertion_result.cpp

@@ -7,8 +7,10 @@
  */
 
 #include <catch2/catch_assertion_result.hpp>
+#include <catch2/internal/catch_stream.hpp>
 
 namespace Catch {
+
     AssertionResultData::AssertionResultData(ResultWas::OfType _resultType, LazyExpression const & _lazyExpression):
         lazyExpression(_lazyExpression),
         resultType(_resultType) {}