When running rubocop, we're getting a lot of deprecation warnings, e.g.:

`EnsureNode#body` is deprecated and will be changed in the next major version of rubocop-ast. Use `EnsureNode#branch` instead to get the body of the `ensure` branch.

(This is with Rubocop 1.45.1.)

We should investigate bumping rubocop to remove those deprecation warnings, though bumping rubocop always has the lovely side-effect of trying to tell us what OTHER things we've been doing wrong all this time, and so may not be a trivial upgrade.