Misc improvements to collections.deque()

Misc improvements to collections.deque()

* Clarified comment on the impact of BLOCKLEN on deque_index
  (with a power-of-two, the division and modulo
   computations are done with a right-shift and bitwise-and).

* Clarified comment on the overflow check to note that
  it is general and not just applicable the 64-bit builds.

* In deque._rotate(), the "deque->" indirections are
  factored-out of the loop (loop invariant code motion),
  leaving the code cleaner looking and slightly faster.

* In deque._rotate(), replaced the memcpy() with an
  equivalent loop.  That saved the memcpy setup time
  and allowed the pointers to move in their natural
  leftward and rightward directions.

See comparative timings at:  http://pastebin.com/p0RJnT5N