experimental: owned objects API (alternative)

[davidhewitt]

This is a fork of #1300 to use PyAny<'py> as an owned pointer, as per discussion in that thread. The PyOwned type is removed.

I haven't implemented any design around PyAnyRaw / PySend / Py yet.

Overall this design feels simpler to work with, so I plan to continue refining this branch over the original one.

The tests aren't all passing yet, but I did run a benchmark:

benchmark	current api	experimental api (PyOwned)	experimental api (PyAny)
bench_call_0	52,895 ns/iter (+/- 6,956)	54,954 ns/iter (+/- 2,175)	51,247 ns/iter (+/- 1,852)
bench_call_method_0	153,977 ns/iter (+/- 9,671)	153,001 ns/iter (+/- 6,288)	141,100 ns/iter (+/- 10,980)
dict_get_item	1,933,300 ns/iter (+/- 328,382)	1,780,027 ns/iter (+/- 79,625)	1,700,250 ns/iter (+/- 52,607)
extract_btreemap	9,538,110 ns/iter (+/- 634,895)	8,688,905 ns/iter (+/- 590,359)	8,201,100 ns/iter (+/- 153,532)
extract_btreeset	8,099,900 ns/iter (+/- 570,799)	7,842,395 ns/iter (+/- 535,694)	7,569,645 ns/iter (+/- 144,288)
extract_hashmap	5,013,300 ns/iter (+/- 483,341)	4,292,595 ns/iter (+/- 358,152)	4,133,425 ns/iter (+/- 231,738)
extract_hashset	5,418,065 ns/iter (+/- 440,259)	4,900,090 ns/iter (+/- 355,389)	4,730,320 ns/iter (+/- 86,596)
iter_dict	1,759,862 ns/iter (+/- 209,509)	1,304,683 ns/iter (+/- 133,726)	1,328,632 ns/iter (+/- 203,382)
iter_list	1,262,143 ns/iter (+/- 76,108)	1,109,227 ns/iter (+/- 52,926)	1,163,203 ns/iter (+/- 86,611)
iter_set	1,257,083 ns/iter (+/- 100,680)	1,037,567 ns/iter (+/- 120,639)	1,069,825 ns/iter (+/- 132,988)
iter_tuple	824,397 ns/iter (+/- 46,234)	753,351 ns/iter (+/- 125,318)	990,398 ns/iter (+/- 93,625)
list_get_item	573,128 ns/iter (+/- 61,386)	531,383 ns/iter (+/- 21,875)	530,595 ns/iter (+/- 37,357)
tuple_get_item	408,859 ns/iter (+/- 25,755)	407,605 ns/iter (+/- 21,552)	520,050 ns/iter (+/- 36,936)

So the conclusion is that it's pretty much the same as the other branch. This is good; I was worried it would be slower!

The only slowdown compared to the other branch is that now tuple iteration / get_item must return an owned PyAny.

[kngwyu]

Thank you for your big efforts in moving this forward!
Could you please consider pushing this branch upstream?
I'm sorry it would force you to do some redundant git&github operations, but I definitely need to play in the code to review such a big PR. Maybe some others can do so.
Now I cannot afford to review this at once but will play with this branch in my break time.

[davidhewitt]

Sure thing, I pushed this as the experimental-api branch to the main PyO3 repo: https://github.com/PyO3/pyo3/tree/experimental-api

Also opened #1308 to track merging that branch.