Add async support

[theCapypara]

Closes #1.

This adds optional and on-by-default async support via maybe-async.

Additionally:

• it bumps the edition to 2021 (and fixes incompatibilies)
• it applies current Clippy lints and cargo fmt
• it bumps byteorder to 1.4
• it versions the Cargo.lock
• converts the example into a proper Cargo example and includes async and sync examples.

#2 should probably also be merged.