field.c, mp.c: Implement the `nb_index' conversion.

field.c, mp.c: Implement the `nb_index' conversion.

This is Yet Another Integer Conversion added in 2.5, for the purpose of
/implicit/ conversions (since strings and other strange things will
convert explicitly to integers).

It's significant because the `bin' builtin (conversion to base-2 string,
added in 2.6) works by applying the `index' conversion rather than
having a special-purpose method like `hex' and `oct'.  (Indeed, Python 3
abolishes these methods, and `hex' and `oct' convert via `index' too.)

So add conversions somewhat judiciously.