Proper support for binary operators, including true division and floor

division.  The basic binary operators now all correctly call the
__rxxx__ variant when they should.

In type_new(), I now make the new type a new-style number unless it
inherits from an old-style number that has numeric methods.

By way of cosmetics, I've changed the signatures of the SLOT<i> macros
to take actual function names and operator names as strings, rather
than rely on C preprocessor symbol manipulations.  This makes the
calls slightly more verbose, but greatly helps simple searches through
the file: you can now find out where "__radd__" is used or where the
function slot_nb_power() is defined and where it is used.