fec: wip: adding block that correlates against an access code and produces a tagged stream ofthe payload (stripped access code).
fec: wip: allowing ber block to be used as a streaming block.
digital: use FFT filters for the correlate_and_sync block.

The known word is interpolated by sps, so it will almost always be a fairly large filter here. Using the FFT filter dramatically reduces the cost of this block.
fec: encoder now outputs bytes to make it more easily integratable with modulators.