Process link statistics from incoming packets.

[ZZ-Cat]

Overview

Before I start working on #69, I need to implement a means of processing the Link Statistics packet that's present in the CRSF stream from the receiver.

Details

In another RC related project of mine, I processed the RSSI, Link Quality, Signal-to-Noise Ratio and Transmitter Power fields of the Link Statistics. I wrote fail-safe logic that was a state machine, and it took priority above extrapolating normal RC channels data from the CRSF data stream from the receiver.
I used arbitrarily set thresholds for RSSI and Link Quality to determine what constituted a "fail-safe" - IE the receiver "losing sight" of the transmitter.

I may base my fail-safe implementation on that.
However, I am leaning more towards making CRSF for Arduino event-based, as evident by my Flight Modes implementation that was introduced in #47.