Various type-related fixes

[jlherren]

No description provided.

[ondrejmirtes]

Thank you, I love it! Do those bugs have already existing GitHub issues or are these new ones?

[jlherren]

I was trying to fix some GitHub issues when I found that I don't entirely understand the fine details of TypeSpecifier and the Type class hierarchy. To study it I ended up writing a fuzz test, where it would randomly choose types A and B and test invariants on them. For example intersect(A, B) should always be the same as remove(A, remove(mixed, B)). And then it started failing for some type combinations :)

I can make a PR for that fuzz test later on.

[ondrejmirtes]

I’d really like these fuzz tests in the test suite, thank you 😊