Python API restructurization & code style improvements (#130)
* Replace tabs with 4 spaces
* Refactor tests
* Rename Python scripts directory to "python"
* Create a separate package for the official Python API
* Move Python inference example to a separate file
* Add missing const
* Refactor extras
* Split rwkv.cpp into smaller files
* Clean up cpp code
* Rename rwkv package to rwkv_cpp
* Add missing type hints
* Rewrite automatic library lookup
* Add compatibility warning
* Fix MacOS build
* Fix MacOS build
