Rust edition 2021 support

[ehuss]

mdbook needs to be updated to support the 2021 edition configuration for Rust. There are several things to do here:

• Update RustEdition and everything that references it (including the javascript code for the playground). In hindsight this probably should have been a free-form string (so it wouldn't need updating each time), but that can't change now. Perhaps that would be good to change for 0.5.
• Update documentation.
• Unfortunately until the edition is stabilized, it requires the -Zunstable-options flag to be passed to rustdoc. I'm thinking maybe just pass that for now, and then remove it when it is stabilized?

[Dylan-DPC-zz]

Unfortunately until the edition is stabilized, it requires the -Zunstable-options flag to be passed to rustdoc. I'm thinking maybe just pass that for now, and then remove it when it is stabilized?

yeah that sounds fine.