Add type hints to all public APIs

[tkukushkin]

fixes #146

I used pyright --verifytypes to check type completeness of public API. Unfortunately I had to move tests to a separate package to exclude them from pyright check.

[charettes]

If that can be of any help I confirm the types provided in this PR addressed the issues we were running into with @stats.Timer decoration of function having loose their return type and preventing type checkers from doing their job.

[chuangfengwang]

Valuable work!