Hash support for row types

Hash support for row types

Add hash functions for the record type as well as a hash operator
family and operator class for the record type.  This enables all the
hash functionality for the record type such as hash-based plans for
UNION/INTERSECT/EXCEPT DISTINCT, recursive queries using UNION
DISTINCT, hash joins, and hash partitioning.

Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: https://www.postgresql.org/message-id/flat/38eccd35-4e2d-6767-1b3c-dada1eac3124%402ndquadrant.com