fixup a couple of warnings
#![cfg_attr(feature = "rt", feature(used))]
#![feature(const_fn)]
#![allow(non_camel_case_types)]
-#![feature(duration_extras)]
extern crate bare_metal;
extern crate cast;
extern crate cortex_m;
pub extern crate embedded_hal as hal;
extern crate void;
-#[macro_use(block)]
pub extern crate nb;
pub extern crate nrf52840;
