Hash support for row types

In [0] it was discussed that hash support for row types/record would be
handy. So I implemented that.

The implementation hashes each field and combines the hash values. Most
of the code structure can be borrowed from the record comparison
functions/btree support. To combine the hash values, I adapted the code
from the array hashing functions. (The hash_combine()/hash_combine64()
functions also looked sensible, but they don't appear to work in a way
that satisfies the hash_func regression test. Could be documented better.)

The main motivation is to support UNION [DISTINCT] as discussed in [0],
but this also enables other hash-related functionality such as hash
joins (as one regression test accidentally revealed) and hash partitioning.

[0]:
https://www.postgresql.org/message-id/flat/52beaf44-ccc3-0ba1-45c7-74aa251cd6ab%402ndquadrant.com#9559845e0ee2129c483b745b9843c571

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Peter Eisentraut http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services