fix: do not require alloc crate (#44)

Using `core::alloc` instead of `alloc::alloc` removes the need to
include `alloc` in this library. This enables users of this crate to use
`Heap` without `alloc` in `no_std`.

Author: haraldh

src/hole.rs

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

src/lib.rs

@@ -12,13 +12,11 @@ extern crate std;
 #[cfg(feature = "use_spin")]
 extern crate spinning_top;
 
-extern crate alloc;
-
-use alloc::alloc::Layout;
-#[cfg(feature = "alloc_ref")]
-use alloc::alloc::{AllocError, Allocator};
 #[cfg(feature = "use_spin")]
 use core::alloc::GlobalAlloc;
+use core::alloc::Layout;
+#[cfg(feature = "alloc_ref")]
+use core::alloc::{AllocError, Allocator};
 use core::mem;
 #[cfg(feature = "use_spin")]
 use core::ops::Deref;