At some point, upgrade to 2021 edition

Rust 1.56 (with support for the 2021 edition) came out last week. Eventually, we should upgrade to require it.

Before we do, we should talk to our downstream users and see what versions of Rust they need to remain buildable with, and use this to make a policy for how long we support older versions of Rust.

Once we have such a policy, we should probably add a CI builder that builds our code with the oldest version of Rust we claim to support.