Allow threads on Index creation

[torymur]

Closes #113

GIL prevents several threads from executing Python bytecode in parallel, so if multiple threads are created, only one thread can execute Python bytecode at any given time.

Release the GIL

To temporarily release the GIL for the cpu-heavy index creation step pyo3's allow_threads seems to be enough to allow other Python threads to run and to get roughly x2+ improvement in multithreaded case, where number of threads is equal to the number of physical cores.

Compare current situation on main branch without allow_threads, tested on my local machine with 6 threads (6 physical cores):

To these benchmarks with allow_threads on index creation and different number of threads:

Of course, amount of threads will still affect the results due to GIL's overall overhead, here as expected: less threads, better results.

Also, separately confirmed equal thread usage with top:

GIL's switch interval affects too

CPU-bound tasks are especially limited by the GIL since they require continuous execution of Python bytecode and default GIL's switch interval is 5ms. To show this effect I added a benchmark with increased interval to 5 secs (the same 6 threads):

These results are much closer to the one-threaded case, than the ones with default GIL's switch interval.