Migrate to Rust 2021 as part of the public testing

Author: hargoniX

Cargo.toml

@@ -10,7 +10,7 @@ authors = ["Andrew Straw <[email protected]>",
            "Matthew Meyer <[email protected]>",
            "Florian Jung <[email protected]>",
            "Matt Ickstadt <[email protected]>"]
-edition = "2018"
+edition = "2021"
 categories = ["embedded", "hardware-support", "no-std"]
 description = "Hardware Abstraction Layer implementation for STM32H7 series microcontrollers"
 keywords = ["arm", "cortex-m", "stm32h7xx", "hal", "embedded-hal"]