Regarding input item look-ups, this reduces asymptotic complexity from
linear to logarithmic time.

Regarding ID look-ups, this reduces insertion and deletion time to
logarithmic. Previously it degraded to linear time because of memcpy()
and memmove() in ARRAY_APPEND and ARRAY_REMOVE macros.

This removes the "all_items" array, and its missing error handlers.

Finally, this adds support for allocating more than INT_MAX items
during the entire lifetime of the VLC instance. (The maximum number of
_concurrent_ items is still INT_MAX, but memory would probably run out
before that is reached.)

Note: Item deletion still requires linear time. And playlist deletion
still consequently requires quadractic time because of the "current"
array.