Speed up open and various fold operations.  This is comprised of
two major changes:
  - replace the visitor fold with an explict crc check.
  - rewrite all folds in terms a new large file fold function that
    adaptively prefetches (and jumps around when prefetching is
    the wrong thing to do) data so that we're operating on larger
    chunks of data, instead of doing a large number of small reads.