Update Rust to 2021 Edition

[slightknack]

It was recently announced that the 2021 edition of Rust was released. I'd like to update this project to the 2021 edition as soon as possible; this will be an intermediate minor release before big-refactor is merged, reflecting some minor changes made since the last release.

This change will also apply to crates under the Veritable umbrella, including aspen, playground, and the experimental compiler backend currently under wraps.