Equip python functions, classes, methods and exceptions with docstrings

[oleksandr-pavlyk]

This PR builds on top of #290. Hence, it should be merged after #290 has been merged.

This PR moves doc-strings for Python objects exposed in cuda.bench namespace from __init__.pyi to implementation file, making doc-strings interactively available to the user. For example, help(cuda.bench.State) would not include doc-strings of the class itself as well as of its methods.

Closes #291