Top/Bottom factor out some common expressions
Add a XXX comment about widing offset.

s_unpack_from to PyCFunction.

- Following Guido's comments, renamed

  * pack_to -> pack_into
  * recv_buf -> recv_into
  * recvfrom_buf -> recvfrom_into

- Made fixes to _struct.c according to Neal Norwitz comments on the checkins
  list.

- Converted some ints into the appropriate -- I hope -- ssize_t and size_t.

loops here).  Assert that size_t is actually big enough, and
that f->size is at least one.  Wrap a long line.