parse/enum: add is_registered test

parse/enum.hpp

@@ -218,6 +218,26 @@ namespace cruft::parse::enumeration {
     }
 
 
+    /// Returns true if a typeidx has been registered for enumeration
+    /// reflection.
+    inline
+    bool is_registered (int idx)
+    {
+        auto const &dat = detail::cache ();
+        auto const  pos = dat.find (idx);
+        return pos != dat.cend ();
+    }
+
+
+    /// Returns true if a typename has been registered for enumation
+    /// reflection.
+    template <typename ValueT>
+    bool is_registered (void)
+    {
+        return is_registered (cruft::typeidx<ValueT> ());
+    }
+
+
     /// Lookup an enumeration value given a string representation.
     ///
     /// The string view is taken by reference and will be modified so that it