WIP: Introducing `cuda.py`: pythonic access to CUDA core functionalities

[leofang]

xref: #70

This is a highly experimental feature currently for preview purposes only, not for production use.

The current focus is centering around a correct, robust, and future-proof design of pythonic CUDA APIs, aiming to boost Python CUDA developers' productivity, among other things. This PR can be considered as a reference/prototype implementation for correctness checks and as a playground, with performance improvements coming next once the design is converged and approved.

More to come in the near future!

TODOs before merging (into the cuda_py branch just forked from main):

• [ ] Expand/update PR description
• [ ] Add README
• [ ] Sync up with the (internal) design doc
• [ ] Polish & check in test files
• [ ] Polish & check in example files
• [x] Update DLpack support
• [ ] Rerun tests with CUDA 11 driver & CTK
• [ ] ~~Fix (?) dependencies on the Python bindings~~
• [x] Add GPUMemoryView prototype
• [ ] ~~Add documentation~~

A separate issue will be created to track needed changes after merging (but before a beta release).