Remove extra list copying

[Julian]

(Filing this mostly as a reminder after taking a quick look at the code)

A bunch of places materialize Python lists of tokens unnecessarily (by calling list() on some iterable).

This isn't a huge deal as we likely will never really need to deal with giant schemas, but it's unnecessary.
We should instead only do this if/when we ever need to (e.g. to maintain state for our additional parsing).