fix the allocator example

importing alloc is a bit weird

examples/allocator.rs

@@ -14,11 +14,12 @@
 #![no_main]
 #![no_std]
 
+extern crate alloc;
 extern crate panic_halt;
 
+use self::alloc::vec;
 use core::alloc::Layout;
 
-use alloc::vec;
 use alloc_cortex_m::CortexMHeap;
 use cortex_m::asm;
 use cortex_m_rt::entry;
@@ -40,6 +41,10 @@ fn main() -> ! {
 
     hprintln!("{:?}", xs).unwrap();
 
+    // exit QEMU
+    // NOTE do not run this on hardware; it can corrupt OpenOCD state
+    debug::exit(debug::EXIT_SUCCESS);
+
     loop {}
 }