Migrate to Rust 2021 edition

[calavera]

There were some issues migrating to 2021 edition. I think we should address the problems and move all the crates to the 2021 editions.

[nmoutschen]

Is there any compelling reason to move to 2021 at the moment?

Currently, the AWS SDK is on 2018 with a MSRV of 1.54.0, so I'd personally prefer to stay there for compatibility reason.

[brainstorm]

smithy-lang/smithy-rs#1388