implemented a decoder using STUArray

  * needs heavier testing
  * mutable arrays
  * uses the two-pass min_dagger: fold the whole row, then tweak each element
  * it has a naive definition of c-hat -- I'm not sure how to optimize that
  * the applicative array idiom isn't supported by the array package. eg repa
    would probably look a lot tidier
  * I also added the 7_20 codes from the old repo (the Wikipedia 6_3 codes
    wouldn't converge for me, even with no noise)