Fix some typechecking errors.

[reitermarkus]

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Also, the StringScanner error will be fixed by sorbet/sorbet#3438.

[issyl0]

Thank you so much for the help, Markus! This makes me feel less overwhelmed! It's good to see the number of errors go down significantly having fixed things!

[vidusheeamoli]

We're down to 9 errors and I don't think we've ever been this low. Thank you @reitermarkus! 👏