This makes sure we always wrap elements in a SubscriptElement, even when there
is only one element. This makes things more regular while still being backwards
compatible with existing creation. The meat of this is in two halves, which can't
be split due to not wanting to break the build between commits. The first half
is just the changes to the parser and updates to tests. This includes a test to
be sure we can still render code that uses old construction types. The second half
is changes to codegen which made assumptions about `Subscript` and demonstrates
the need to make this change in the first place. This includes a fix to
`CSTNode.with_deep_changes` type to make it more correct and also more usable in
transforms without additional type assertions.
I noticed that the typed visitors codegen was creating messy types such as Union[SingleType]. We have a clean-up for Union[SingleType] snuck into gen_matcher_classes. So, lets remove that snuck-in clean-up from gen_matcher_classes and apply it globally to all codegen. This makes its purpose a lot more obvious, while helping decouple necessary codegen from prettifying/simplifying transforms. It also cleans up typed visitors, so that's a bonus.
