ruat version 0.2.1
The idea here is to make something like dump1090, but for UAT.

We now have dump978, and ruat mostly offers historic interest. See:
 https://github.com/mutability/dump978

The typical output looks like this (uplink lines are very long, and
this example is truncated!):

  +3215fb689a02a090000000000000000000000 ....... 00000000000000000000000000;
  -0aa4c7aa3306f968e3f6149810522d606f2f5c012da2040b8a9502a0001510000000;
  -0aa4c7aa3306ef68e3b6149810462d602f2f5c012da2040b8a9502a0001510000000;
  Samples 20840448 dT 10003402 Bits 4246373 Maxlen 4452 Syncs a:6 u:6

Lines staring with + and - are Uplink and ADS-B messages. Everything
else are periodic messages about the performance of ruat. The most
interesting is Maxlen, the number of consequetive bits. It should be
4452 for a valid Uplink packet. If you get less, it means the dongle
isn't keeping sync or the antenna is not good enough. If you see 15 to 20,
it means you aren't receiving anything at all.

If you are away from an airplane, make sure to test with maximum gain.
Run rtl_test to identify the maximum gain, then set it with -g XX
(-g 42 for Elonics, -g 49 for R820T).

TODO
 - switch to fixed point, we're still on 11% CPU
 - precalc arc-tangents once and link to them at build time
 - learn from Oliver and get rid of phi buffers entirely; calculate phi,
   compare with A/U code, unroll the bits with a known odd/even.
 - get rid of obfuscating libraries and talk directly to libusb