Move some compile-time dispatch to runtime

The runtime performance is likely to be negligible,
but compile times need every improvement they can get.

include/internal/catch_evaluate.cpp

@@ -0,0 +1,38 @@
+/*
+ *  Created by Martin on 01/08/2017.
+ *
+ *  Distributed under the Boost Software License, Version 1.0. (See accompanying
+ *  file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
+ */
+
+#include "catch_evaluate.hpp"
+
+#include "catch_enforce.h"
+
+namespace Catch {
+namespace Internal {
+
+    const char* operatorName(Operator op) {
+        switch (op) {
+            case IsEqualTo:
+                return "==";
+            case IsNotEqualTo:
+                return "!=";
+            case IsLessThan:
+                return "<";
+            case IsGreaterThan:
+                return ">";
+            case IsLessThanOrEqualTo:
+                return "<=";
+            case IsGreaterThanOrEqualTo:
+                return ">=";
+            default:
+                CATCH_ERROR("Attempting to translate unknown operator!");
+        }
+    }
+
+    // nullptr_t support based on pull request #154 from Konstantin Baumann
+    std::nullptr_t opCast(std::nullptr_t) { return nullptr; }
+
+} // end of namespace Internal
+} // end of namespace Catch