mpmc: use heapless API for crossbeam impl

Converts the `crossbeam::ArrayQueue` impl for MPMC to use a
`heapless`-compatible API.

Author: 0xllx0

src/mpmc.rs

@@ -68,6 +68,23 @@
 //!
 //! [bounded MPMC queue]: http://www.1024cores.net/home/lock-free-algorithms/queues/bounded-mpmc-queue
 
+use crate::storage::ViewStorage;
+
+#[cfg(not(feature = "portable-atomic"))]
+use core::sync::atomic;
+#[cfg(feature = "portable-atomic")]
+use portable_atomic as atomic;
+
+#[cfg(feature = "mpmc_large")]
+type AtomicTargetSize = atomic::AtomicUsize;
+#[cfg(not(feature = "mpmc_large"))]
+type AtomicTargetSize = atomic::AtomicU8;
+
+#[cfg(feature = "mpmc_large")]
+type UintSize = usize;
+#[cfg(not(feature = "mpmc_large"))]
+type UintSize = u8;
+
 #[cfg(feature = "mpmc_crossbeam")]
 pub mod crossbeam_array_queue;
 #[cfg(feature = "mpmc_crossbeam")]
@@ -77,3 +94,8 @@ pub use crossbeam_array_queue::*;
 mod original;
 #[cfg(not(feature = "mpmc_crossbeam"))]
 pub use original::*;
+
+/// A [`Queue`] with dynamic capacity.
+///
+/// [`Queue`] coerces to `QueueView`. `QueueView` is `!Sized`, meaning it can only ever be used by reference.
+pub type QueueView<T> = QueueInner<T, ViewStorage>;