C function proposal redux

I propose we modify C functions for 7.2.

( I'll volunteer to do as much as I can figure out ;-)

(1) C functions should be able to return multiple values.

(2) A setup and breakdown function should be able to be called
surrounding the query set in which a function is called. This allows
constructors and destructors.

(3) A function should be able to tell Postgres how to use it. For
instance:

select * from table where column = function();

Should be able to instruct Postgres to either take the value returned
and search that one value (allowing index match against the value), or
perform a table scan against the function each time. Both behaviors are
important. Currently a function seems to force a table scan.

Estimates:
1 may be difficult. 2 should be easy enough. 3, depending on the code
dependencies, could either be very hard or easy. (my guess is that it
would be hard)

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