GeezerVison

These are included in the docs (newtypes_tutorial.rst), which redundantly define most of those in the prose, and have some notes that reference the casts. Could you change those as well?
(Perhaps there's a way to consolidate this with something like Sphinx's :start-at:, but that might be a distraction right now. In any case, some of the actual prose does need to be updated.)

I observed that I incorrectly called a function as well, so thanks Petr for pointing out the docs.

@encukou feel free to merge this one tomorrow or whenever you want, I'm not on my dev session so I can only edit from the web UI.

Thanks!
FWIW, there's also newtypes.rst with some UB examples.