Add support for AVR.

In order to support AVR, the following changes were made:

 - AVR was added to the list of architectures supported by atomic-polyfill.
 - SortedLinkedList was modified to account for the fact that usize is
   differently sized on different architectures (16 bits on AVR).

Author: jeremysalwen

CHANGELOG.md

@@ -10,6 +10,7 @@ and this project adheres to [Semantic Versioning](http://semver.org/).
 ### Fixed
 
 ### Added
+- Added support for AVR architecture.
 
 ### Changed
 

src/sorted_linked_list.rs

@@ -95,7 +95,7 @@ where
 
 // Internal macro for generating indexes for the linkedlist and const new for the linked list
 macro_rules! impl_index_and_const_new {
-    ($name:ident, $ty:ty, $new_name:ident, $max_val:literal) => {
+    ($name:ident, $ty:ty, $new_name:ident, $max_val:expr) => {
         /// Index for the [`SortedLinkedList`] with specific backing storage.
         #[derive(Debug, Clone, Copy, PartialEq, Eq, PartialOrd, Ord)]
         pub struct $name($ty);
@@ -178,9 +178,9 @@ macro_rules! impl_index_and_const_new {
     };
 }
 
-impl_index_and_const_new!(LinkedIndexU8, u8, new_u8, 254); // val is 2^8 - 2 (one less than max)
-impl_index_and_const_new!(LinkedIndexU16, u16, new_u16, 65_534); // val is 2^16 - 2
-impl_index_and_const_new!(LinkedIndexUsize, usize, new_usize, 4_294_967_294); // val is 2^32 - 2
+impl_index_and_const_new!(LinkedIndexU8, u8, new_u8, { u8::MAX as usize - 1 });
+impl_index_and_const_new!(LinkedIndexU16, u16, new_u16, { u16::MAX as usize - 1 });
+impl_index_and_const_new!(LinkedIndexUsize, usize, new_usize, { usize::MAX - 1 });
 
 impl<T, Idx, K, const N: usize> SortedLinkedList<T, Idx, K, N>
 where