Replace unidiomatic 'extern crate' to 'use x as _'

examples/exception.rs

@@ -10,7 +10,7 @@
 #![no_main]
 #![no_std]
 
-extern crate panic_halt;
+use panic_halt as _;
 
 use cortex_m::peripheral::syst::SystClkSource;
 use cortex_m::Peripherals;