Use portable-atomic

This should not have run-time impact on platforms that support atomic
boolean compare-and-swap, but enables its use on microcontrollers where
the user configures portable-atomic in such a way that atomics can be
emulated (eg. through disabling interrupts).

Cargo.toml

@@ -12,3 +12,4 @@ documentation = "https://docs.rs/try-lock"
 readme = "README.md"
 
 [dependencies]
+portable-atomic = { version = "1", default-features = false, features = ["require-cas"] }

src/lib.rs

@@ -46,10 +46,13 @@
 #[cfg(test)]
 extern crate core;
 
+extern crate portable_atomic;
+
 use core::cell::UnsafeCell;
 use core::fmt;
 use core::ops::{Deref, DerefMut};
-use core::sync::atomic::{AtomicBool, Ordering};
+use portable_atomic::AtomicBool;
+use core::sync::atomic::Ordering;
 use core::marker::PhantomData;
 
 /// A light-weight lock guarded by an atomic boolean.