uni-t-ut181a: implement device driver for the UNI-T UT181A multimeter

uni-t-ut181a: implement device driver for the UNI-T UT181A multimeter

Extend the previously introduced skeleton driver for UNI-T UT181A. Introduce
support for the full multimeter's protocol as it was documented by the ut181a
project. Which covers the retrieval of live readings, saved measurements, and
recordings, in all of the meter's modes and including relative, min/max, and
peak submodes. This implementation also parses compare mode (limits check)
responses, although it cannot express the result in terms of the session feed.

Announce the device as a multimeter as well as a thermometer, it supports
up to two probes including difference mode. When in doubt, prefer usability
over feature coverage (the driver side reflects all properties of the meter,
but not all features can get controlled by the driver). The probe routine
requires that users specify the serial port, and enable serial communication
on the meter.

Several TODO items remain. Comments in the driver code discuss limitations
of the current implementation, as well as cases where the meter's features
don't map well to sigrok's internal presentation. This implementation also
contains (optional, off by default) diagnostics for research on the serial
protocol.