v0.3.1

Fixed

• Fix sn_std_allocator realloc losing the payload when the block moved. It
  reallocates a raw block, re-aligns and returns the new address, but only
  copied anything when the new address happened to match the old one. Every
  block now starts with a header holding its size, and the bytes are moved from
  the old offset to the new one
• Fix sn_std_allocator realloc writing the new offset over the bytes it was
  about to copy. The offset goes in the byte below the payload, so a payload
  that shifts upwards lands on top of its own contents. The move happens first
  now
• Reject a size and alignment whose raw block would not fit, rather than
  wrapping around and handing back a block smaller than was asked for