This library provides implementations of concurrent FIFO queues (for both
general boxed and primitive unboxed values) that are fast, perform well
under contention, and offer a Chan-like interface. The library may be of
limited usefulness outside of x86 architectures where the fetch-and-add
instruction is not available.

We export several variations of our design; some support additional
functionality while others try for lower latency by removing features or
making them more restrictive (e.g. in the Unboxed variants).

* Unagi: a general-purpose near drop-in replacement for Chan.
* Unagi.Unboxed: like Unagi but specialized for primitive types; this may
  perform better if a queue grows very large.
* Unagi.Bounded: a bounded variant with blocking and non-blocking writes,
  and other functionality where a notion of the queue's capacity is
  required.
* Unagi.NoBlocking: lowest latency implementations for when blocking reads
  aren't required.
* Unagi.NoBlocking.Unboxed: like Unagi.NoBlocking but for primitive types.