Teach estimate_array_length() to use statistics where available.

If we have DECHIST statistics about the argument expression, use
the average number of distinct elements as the array length estimate.
(It'd be better to use the average total number of elements, but
that is not currently calculated by compute_array_stats(), and
it's unclear that it'd be worth extra effort to get.)

To do this, we have to change the signature of estimate_array_length
to pass the "root" pointer.  While at it, also change its result
type to "double".  That's probably not really necessary, but it
avoids any risk of overflow of the value extracted from DECHIST.
All existing callers are going to use the result in a "double"
calculation anyway.

Paul Jungwirth, reviewed by Jian He and myself

Discussion: https://postgr.es/m/CA+renyUnM2d+SmrxKpDuAdpiq6FOM=FByvi6aS6yi__qyf6j9A@mail.gmail.com

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