Unit tests

Some of the functions have unit tests, which are included in the unittests folder. The tests can be run on Linux using simply make, which runs the test for all CompilationOptions.

Tests can also be run on other platforms by linking fix16_unittests.c with the fix16 library. The unittests will output a report to stdout.

The tests perform some corner case tests with hardcoded expected results and also automated tests using an array of test numbers, comparing against the platform double implementation. The tests automatically take into account ie. if rounding is disabled, so they should pass in any configuration. They will however not pass on AVR, because the 32bit-only double there is not accurate enough (so the reference results are actually wrong).