By dispatching to long int methods.

This should fix issue #171

Long int is something which doesn't fit into SMALL_INT partion of
machine_int_t. But it's also something which doesn't fit into
machine_int_t in the first place.

We likely should make mp_obj_new_int() inline, and rely on its
encapsulated check rather than inline checks everywhere explicitly.

Also, parser for big small int values is still broken.

Currently it would report "operation not supported" which is confusing. Overall,
this is thinko leading to undefined behavior.

Still need to make built-ins by these names, and write tests.

If step is not constant, in first approximation, we can't apply optimization,
(well, we could, but need a special case for this).