Added support for pickling indices, support for PEP-517 and PEP-518 building, small speedups, bugfixes and documentation improvements.
The setup.py file now resides in the root.

Many thanks to @dbespalov, @dyashuni, @groodt, @uestc-lfs, @vinnitu, @fabiencastan, @JinHai-CN, @js1010!