rt-history: An RT-safe history log with error checking

This is a bounded wait-free thread synchronization primitive which allows you to record the time evolution of some data on one thread and be able to query the last N data points on other threads.

By definition of wait-free synchronization, the producer and consumer threads cannot wait for each other, so in bad conditions (too small a buffer size, too low a thread's priority, too loaded a system...), two race conditions can occur:

• The producer can be too fast with respect to consumers, and overwrite historical data which a consumer was still in the process of reading. This buffer overrun scenario is reported, along with the degree of overrun that occurred, which can be used to guide system parameter adjustments so that the error stops occurring.
• The producer can be too slow with respect to consumers, and fail to write a sufficient amount of new data inbetween two consumer readouts. The precise definition of this buffer underrun error is workload-dependent, so we provide the right tool to detect it in the form of a latest data point timestamp, but we do not handle it ourselves.

```rust let (mut input, output) = RTHistory::::new(8).split();

let inbuf = [1, 2, 3]; input.write(&inbuf[..]);

let mut outbuf = [0; 3]; asserteq!(output.read(&mut outbuf[..]), Ok(3)); asserteq!(inbuf, outbuf); ```