Provide a polyfill over std::to_string

Android apparently does not support `std::to_string`, so we add a
small polyfill over it. Right now only the ULP matcher uses it,
but we have had plans to use it in `StringMaker<int>` and friends,
as it performs a lot better than `std::stringstream` based
stringification on MSVC.

See #1280 for more details

include/internal/catch_to_string.hpp

@@ -0,0 +1,28 @@
+/*
+ *  Created by Martin on 9/5/2018.
+ *
+ *  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)
+ */
+#ifndef TWOBLUECUBES_CATCH_TO_STRING_H_INCLUDED
+#define TWOBLUECUBES_CATCH_TO_STRING_H_INCLUDED
+
+#include <string>
+
+#include "catch_compiler_capabilities.h"
+#include "catch_stream.h"
+
+namespace Catch {
+    template <typename T>
+    std::string to_string(T const& t) {
+#if defined(CATCH_CONFIG_CPP11_TO_STRING)
+        return std::to_string(t);
+#else
+        ReusableStringStream rss;
+        rss << t;
+        return rss.str();
+#endif
+    }
+} // end namespace Catch
+
+#endif // TWOBLUECUBES_CATCH_TO_STRING_H_INCLUDED