Miscellaneous refactorings

* Add comment to the maxlen structure entry about the meaning of maxlen == -1
* Factor-out code common to deque_append(left) and deque_extend(left)
* Factor inner-loop in deque_clear() to use only 1 test per loop instead of 2
* Tighten inner-loops for deque_item() and deque_ass_item() so that the
  compiler can combine the decrement and test into a single step.

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