Rename catch_stream -> catch_reusable_string_stream

After everything else was split out, this name much reflects the
actual contents of the file(s).

src/catch2/catch_approx.cpp

@@ -7,7 +7,7 @@
 // SPDX-License-Identifier: BSL-1.0
 #include <catch2/catch_approx.hpp>
 #include <catch2/internal/catch_enforce.hpp>
-#include <catch2/internal/catch_stream.hpp>
+#include <catch2/internal/catch_reusable_string_stream.hpp>
 
 #include <cmath>
 #include <limits>