Robust packet reading.
* Read characters until a start-of-packet character is read.
* Fix incorrect packet length for LocationPacket.
This fixes the library issues that caused adafruit/circuitpython#1568. The CircuitPython issues were fixed by
adafruit/circuitpython#1518.
To use in CircuitPython, simply install the Adafruit CircuitPython bundle.
Read the docs for info on how to use it.