a little reorg and polish here and there
first draft of the Repa ECC

  * I ignored the representation parameter--arbitrarily using U annotations
    (via the type synonyms) and arbitrarily inserting computeS to fix type
    errors--so there is almost certainly easy improvements to make

  * on the 7/20 code, it's about twenty times slower than the mutable
    array-based version

  * it is very slow on the 4K/7K code, and immediately takes up 5GB of memory