Piece size selection reform

[casey]

• Support human readable piece sizes using SI units (kibi, gibi, etc.) defaulting to bytes if unmarked
• Complain if piece size is less than 16KiB, for compatibility with BTv2 spec. Allow users to pass a flag to override. (Possibly a generic --allow CHECK flag that can be reused for other things.)
• Complain if piece sizes are not a power of two. Allow overriding
• Complain if piece size is zero
• Come up with a reasonable default if piece size isn't supplied, depending on the amount of data in the torrent (1200 - 2200 pieces is recommended, max 16MiB min 16KiB), pyrocore uses:

  piece_size_exp = int(math.log(total_size) / math.log(2)) - 9
  piece_size_exp = min(max(15, piece_size_exp), 24)
  piece_size = 2 ** piece_size_exp