Lexer and grammar rules for a RETURNING clause on DELETE/INSERT/UPDATE.

Lexer and grammar rules for a RETURNING clause on DELETE/INSERT/UPDATE.
Actually making this work, though, will involve a lot more code which will
likely slow down processing for the common case where there is no
RETURNING clause.  Furthermore, RETURNING seems to be of limited usefulness
and it is not standard SQL.  So we abandon it here.  These experimental
changes are parked in a branch as an historical reference.  If circumstances
changes, we might take up the cause again some day.

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