Fix import paths for Alloc/AllocErr/Layout on latest nightly

Fixes #14

Author: phil-opp

src/hole.rs

@@ -1,4 +1,4 @@
-use alloc::allocator::{AllocErr, Layout};
+use alloc::alloc::{AllocErr, Layout};
 use core::mem::size_of;
 use core::ptr::NonNull;
 

src/lib.rs

@@ -12,7 +12,7 @@ extern crate spin;
 
 extern crate alloc;
 
-use alloc::allocator::{Alloc, AllocErr, Layout};
+use alloc::alloc::{Alloc, AllocErr, Layout};
 use core::alloc::{GlobalAlloc};
 use core::mem;
 #[cfg(feature = "use_spin")]