Update to Rust 2021 edition

This commit updates us to use the Rust 2021 edition. I ran cargo fix
--edition with the appropriate additional target flags etc. for several
boards (Imix, OT, hail) and no changes were needed anywhere.

Author: hudson-ayers

libraries/tock-register-interface/Cargo.toml

@@ -9,7 +9,7 @@ readme = "README.md"
 keywords = ["tock", "embedded", "registers", "mmio", "bare-metal"]
 categories = ["data-structures", "embedded", "no-std"]
 license = "MIT/Apache-2.0"
-edition = "2018"
+edition = "2021"
 
 [badges]
 travis-ci = { repository = "tock/tock", branch = "master" }