The main challenge with making the class version fast is that you pay the
additional function call overhead for the call to `[](). I think inlining does
not work unless you are inlining something from within the same class (or
parent maybe). It would be nice if the compiler would generate a fast path
which assumes that your loop always calls `[]() in your class 'amapping', in
that case it would be possible to optimize the function call in a way similar
to what the 'fast_call' branch does with map(), etc.
