Improve parse representation for MERGE

Improve parse representation for MERGE

Separation of parser data structures from executor, as
requested by Tom Lane. Further improvements possible.

While there, implement error for multiple VALUES clauses via parser
to allow line number of error, as requested by Andres Freund.

Author: Pavan Deolasee

Discussion: https://www.postgresql.org/message-id/CABOikdPpqjectFchg0FyTOpsGXyPoqwgC==OLKWuxgBOsrDDZw@mail.gmail.com