Code style fixes and guidelines

[fenhl]

This PR includes various code style fixes that have been accumulating on my fork for a while. Most things are minor, like fixing typos in variable names, adjusting whitespace, or removing unnecessary parentheses, but there's also a couple instances where code duplication is removed, and replacing lists with tuples in the Python code should be a (very small) performance improvement.

I've also updated CI.py to include a few more checks and files. Where adding automated checks wasn't feasible, I've tried to formalize the changes I've made and some other existing conventions into a new document Notes/code-style.md. Here's my rationale for some of the guidelines that might be more controversial:

• Commented-out code: Dead code adds bloat to the codebase without accomplishing anything. Commented-out code is always dead, so it must serve as documentation to be useful.
• Parentheses around and inside or: This formalizes existing practice in the codebase, especially in the logic files.
• Uppercase hex digits: This is the opposite of what I'm used to, since lowercase letters tend to be more distinct from digits. But uppercase is what's used in most of the codebase, so I'm adding this guideline so people like me don't accidentally introduce inconsistent style.
• Pointer variables: This reflects the fact that in int *a, b, b is of type int, not int *. Even though we currently don't use multiple variable declarations like this (at least as far as I'm aware), not being aware of this quirk of the language can lead to subtle bugs, so I think it's best not to obscure it with the formatting. Overruled by the dev team.
• Trailing comma: This improves readability of diffs where elements are added or removed from such a list.
• Tuples instead of lists: This is for performance reasons. If a tuple consists of only literals, the Python parser will in turn treat it as a literal, which means that it will only ever be constructed once per program execution, while an equivalent list would be constructed each time that line of code runs.

[fenhl]

Removing the code duplication from Hints.hint_dist_keys revealed a bug where the important_check hint type was missing from that set. Fixing it caused unit tests to fail because some plandos are missing that hint type from their custom hint distro and important_check wasn't one of the hint types that were filled in with default values for backwards compatibility. I've decided to add this default for all hint types rather than special-casing just the newer ones. This will allow hint distros to be less verbose by completely omitting all unused hint types.

[flagrama]

What about a .clang-tidy config file so I can just feed that to my editor and have it ensure the correct style? (and that it won't undo style changes) I don't know how much it can do, but something like the pointer declaration syntax is probably something it handles.

[cjohnson57]

What about a .clang-tidy config file so I can just feed that to my editor and have it ensure the correct style? (and that it won't undo style changes) I don't know how much it can do, but something like the pointer declaration syntax is probably something it handles.

That would be a good thing to have, but I won't require it as part of this initial PR

[cjohnson57]

I'm good to merge this as soon as we have any 8.0 bugs taken care of.