Call the generator helper method directly instead of calling
`PyIter_Next` when calling a native generator from a native function.
This way we can avoid raising StopIteration when the generator is
exhausted. The approach is similar to what I used to speed up calls
using await in #19398. Refer to that PR for a more detailed explanation.

This helps mostly when a generator produces a small number of values,
which is quite common.

This PR improves the performance of this microbenchmark, which is a
close to the ideal use case, by about 2.6x (now 5.7x faster than
interpreted):
```
from typing import Iterator

def foo(x: int) -> Iterator[int]:
    for a in range(x):
        yield a

def bench(n: int) -> None:
    for i in range(n):
        for a in foo(1):
            pass

from time import time
bench(1000 * 1000)
t0 = time()
bench(50 * 1000 * 1000)
print(time() - t0)
```