pgsql: Introduce TupleHashTableHash() and LookupTupleHashEntryHash().

Introduce TupleHashTableHash() and LookupTupleHashEntryHash().

Expose two new entry points: one for only calculating the hash value
of a tuple, and another for looking up a hash entry when the hash
value is already known. This will be useful for disk-based Hash
Aggregation to avoid recomputing the hash value for the same tuple
after saving and restoring it from disk.

Discussion: https://postgr.es/m/37091115219dd522fd9ed67333ee8ed1b7e09443.camel%40j-davis.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4eaea3db150af56aa2e40efe91997fd25f3b6d73

Modified Files
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src/backend/executor/execGrouping.c | 108 ++++++++++++++++++++++++++----------
src/include/executor/executor.h     |   5 ++
2 files changed, 83 insertions(+), 30 deletions(-)