Split taichi C++ library into separate python package.

[cloudhan]

proposed feature
Jax has two pacakge jax and jaxlib
Code that need C++ compilation goes into jaxlib. This enables contributors not familiar with C++ to contribute the project easier. E.g add examples, fix python level bugs. So I'd suggest taichi split the python package as jax does.

[k-ye]

Thanks for the advice, this will be super useful!