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box: speed up select/pairs with pagination by reusing ibuf

The normalize_position() internal function is called by select/pairs to
prepare a position specified by the caller in the options.after argument
to be passed to C over FFI. The function encodes the position passed as
a table or a tuple in a temporary buffer obtained with cord_ibuf_take().
The problem is the cord buffer is already taken by the calling function
(select/pairs) to encode the search key. As a result, cord_ibuf_take()
allocates a new temporary buffer with malloc.

Let's avoid this extra buffer allocation by passing the taken buffer to
normalize_position(). Let's also remove the out arguments because they
are always set to iterator_pos and iterator_pos_end and instead set
iterator_pos and iterator_pos_end directly. To make this behavior clear,
we rename normalize_position() to iterator_pos_set(). We also remove
box.error() from this function and instead make it return true/false so
that the caller can release the buffer on failure.

perf/lua/box_select.lua test results:

                      base          patched
              get_0    159      153 (-  3%)
              get_1    243      243 (+  0%)
           select_0    227      227 (+  0%)
           select_1    343      336 (-  2%)
           select_5   2323     2305 (-  0%)
          select_10   6832     6866 (+  0%)
     select_after_0    687      558 (- 18%)
     select_after_1    746      524 (- 29%)
     select_after_5   2917     2704 (-  7%)
    select_after_10   5033     4979 (-  1%)
 select_fetch_pos_0    239      233 (-  2%)
 select_fetch_pos_1    418      413 (-  1%)
 select_fetch_pos_5   2492     2496 (+  0%)
select_fetch_pos_10   5121     5179 (+  1%)
            pairs_0    320      314 (-  1%)
            pairs_1   1060     1054 (-  0%)
            pairs_5   1466     1459 (-  0%)
           pairs_10   3749     3913 (+  4%)
      pairs_after_0    788      670 (- 14%)
      pairs_after_1   1813     1611 (- 11%)
     ... (continued)

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