Bugfixes and improvements. Many thanks to:
@marekhanus for fixing the missing arguments, adding support for python 3.8, 3.9 in Travis, improving python wrapper and fixing typos/code style;
@apoorv-sharma for fixing the bug int the insertion/deletion logic;
@shengjun1985 for simplifying the memory reallocation logic;
@TakaakiFuruse for improved description of add_items;
@psobot for improving error handling;
@ShuAiii for reporting the bug in the python interface.