+ Added interface docs.
+ Gave it a new "newsize" argument, for when the caller can gain speed
  by knowing in advance how many additional elements are needed.  This
  isn't used yet, but will be by multiunion and planned "lopsided merge"
  optimizations (the intent is to grow the bucket big enough in one
  shot, then memcpy a large slice into the bucket).
+ When doubling the size, now sets a MemoryError exception on int
  overflow.