CLN: rename reduce-->do_reduce
The current name makes it difficult to grep for.

Assorted cleanups; added type annotations in a few places.
> I would also change _reduce in ExtensionArray to match

need this too The `_reduce` is part of the EA interface, so we cannot just rename that.

Besides, that seems like to defeat Brock's purpose to be able to search for a specific function (and thus not having multiple functions or methods with the same name that do a different thing) We have _reduce defined in a bunch of places, renaming it should be a separate discussion can you rebase rebased+green Thanks!