cast: account for pointers-to-const in ffs

cast.hpp

@@ -188,7 +188,17 @@ namespace cruft::cast {
 #if defined(COMPILER_GCC)
 #pragma GCC diagnostic ignored "-Wcast-function-type"
 #endif
-        return reinterpret_cast<DstT> (src);
+        // If we're casting a pointer then we may need to first use a
+        // const_cast if we want to avoid any warnings about casting, eg in
+        // the case of `const ValueT*` to `ValueT*`
+        if constexpr (std::is_pointer_v<SrcT>) {
+            using value_type = std::remove_pointer_t<SrcT>;
+            return reinterpret_cast<DstT> (
+                const_cast<std::remove_cv_t<value_type>*> (src)
+            );
+        } else {
+            return reinterpret_cast<DstT> (src);
+        }
 #pragma GCC diagnostic pop
     }