Zeroizing allocator (#154)

* Add a zeroizing allocator

* Also implement the allocator API

* Add misisng license file to zalloc

* Slight change to zalloc description

common/zalloc/src/lib.rs

@@ -0,0 +1,46 @@
+#![cfg_attr(docsrs, feature(doc_cfg))]
+#![cfg_attr(docsrs, feature(doc_auto_cfg))]
+#![cfg_attr(feature = "allocator", feature(allocator_api))]
+
+//! Implementation of a Zeroizing Allocator, enabling zeroizing memory on deallocation.
+//! This can either be used with Box (requires nightly and the "allocator" feature) to provide the
+//! functionality of zeroize on types which don't implement zeroize, or used as a wrapper around
+//! the global allocator to ensure *all* memory is zeroized.
+
+use core::{
+  slice,
+  alloc::{Layout, GlobalAlloc},
+};
+
+use zeroize::Zeroize;
+
+/// An allocator wrapper which zeroizes its memory on dealloc.
+pub struct ZeroizingAlloc<T>(pub T);
+
+#[cfg(feature = "allocator")]
+use core::{
+  ptr::NonNull,
+  alloc::{AllocError, Allocator},
+};
+#[cfg(feature = "allocator")]
+unsafe impl<T: Allocator> Allocator for ZeroizingAlloc<T> {
+  fn allocate(&self, layout: Layout) -> Result<NonNull<[u8]>, AllocError> {
+    self.0.allocate(layout)
+  }
+
+  unsafe fn deallocate(&self, mut ptr: NonNull<u8>, layout: Layout) {
+    slice::from_raw_parts_mut(ptr.as_mut(), layout.size()).zeroize();
+    self.0.deallocate(ptr, layout);
+  }
+}
+
+unsafe impl<T: GlobalAlloc> GlobalAlloc for ZeroizingAlloc<T> {
+  unsafe fn alloc(&self, layout: Layout) -> *mut u8 {
+    self.0.alloc(layout)
+  }
+
+  unsafe fn dealloc(&self, ptr: *mut u8, layout: Layout) {
+    slice::from_raw_parts_mut(ptr, layout.size()).zeroize();
+    self.0.dealloc(ptr, layout);
+  }
+}