We'll want to reuse these for additional modules in this crate.

This is a VirtualAddressSpaceIndependent implementation of a channel.
The intent is to use this to replace `BinarySpinningSem` and `IPCData`
to communicate with managed threads from ManagedThread in Shadow.

The previous implementation had potential undefined behavior due to
optimistically updating state without checking the previous state.

We do away with the `fetch_add` and `fetch_sub` tricks here (with sanity
checks after the fact), and instead use e.g. `compare_exchange`.
This results in additional atomic operations in the "happy" path, but is
easier to understand, especially in failure cases.

This is more general and more closely aligned with `std::mpsc::channel`
terminology.