PyTuple: use fast macros on !abi3

[birkenfeld]

They were already defined, but not used so far. For abi3 they now forward
to the C functions.

Question: should these be defined at all for abi3, or should I add the conditional code in the PyTuple impl?

[davidhewitt]

It looks like these macros are only defined in https://github.com/python/cpython/blob/main/Include/cpython/tupleobject.h, i.e. for the unlimited abi.

So I'd vote for doing the same for our ffi module - only define these macros for the unlimited abi and write the conditional code inside our tuple code.

If there's lots of places in the code where this happens, can always add tiny #[inline] pub(crate) fn pytuple_set_item helpers which are defined differently for each #[cfg] and call to the relevant ffi function.

[birkenfeld]

So far, these were each only used in one place, so I went and added the conditional there.

[davidhewitt]

LGTM, nice one!

FWIW, I get ~2% speedup with tuple_get_item benchmark on this branch, and ~8% speedup on a tuple_new benchmark I'll push in a minute.