Simplify crypto IOV helpers

Simplify crypto IOV helpers

Expand the concept of an IOV block state into a cursor which remembers
the IOV set being iterated over, the block size, and both input and
output positions.  Eliminate the no-copy inline block getter for now,
but provide helpers to grab contiguous chains of blocks from a cursor.
Also provide an inline helper to sum the total length of an iov chain.