Type annotations

[jack-fs]

I propose this issue as a place to consolidate high-level typing-related discussion/planning/commits.

The code-base is partially type-annotated; it has been very useful for documentation and testing (in particular when using hypothesis) so far.

Though it would be premature (and a time-sink) at the moment, I imagine that once the code-base settles we could potentially add mypy type-checking to the CI.

[jack-fs]

It may be worth investigating different type checkers, e.g. pytype, which sells itself as focussing on type inference rather than focussing on gradual typing, and is more lenient than mypy.
This combination seems likely to add less unnecessary noise (type annotation noise and type warning noise) to any refactor.

A comparison paper: https://www.cs.rpi.edu/~milanova/docs/dls2020.pdf

Also, for a more PEP-up-to-date, pyright