Constriction

Minor improvements to error messages and documentation.

This release downgrades the dependency on numpy from "^1.20" to "^1.19". This should fix issues on Google colab.

Greatly simplified and extended the Python API and improved its [documentation](https://bamler-lab.github.io/constriction/apidoc/python/). Only minimal changes to the [Rust API](https://docs.rs/constriction/latest/constriction/).

While this release technically introduces breaking changes to the Python API (hence the jump from v0.1.5 to v0.2.0), you're likely to be able to upgrade without problems. Most methods that were made obsolete by the new API have only been deprecated so far, and they will print a warning message with upgrade instructions.

Some old Linux systems and new Mac systems (with ARM chips) on which constriction for Python did not work before should now work starting with this release due to a more careful CI setup. Apart from this, no change in functionality compared to previous release.

- First test of releasing cross-compiled python extension modules for Apple ARM chips (not yet tested on real hardware).
- Implement `Into<Vec<Word>>` for appropriate coders.
- Improve python API documentation (mainly related to `CustomModel`).

No changes in functionality, just slight improvements to python documentation, especially for `ChainCoder`.