Avoid premature free of pass-by-reference CALL arguments.

Prematurely freeing the EState used to evaluate CALL arguments led, in some
cases, to passing dangling pointers to the procedure.  This was masked in
trivial cases because the argument pointers would point to Const nodes in
the original expression tree, and in some other cases because the result
value would end up in the standalone ExprContext rather than in memory
belonging to the EState --- but that wasn't exactly high quality
programming either, because the standalone ExprContext was never
explicitly freed, breaking assorted API contracts.

In addition, using a separate EState for each argument was just silly.

So let's use just one EState, and one ExprContext, and make the latter
belong to the former rather than be standalone, and clean up the EState
(and hence the ExprContext) post-call.

While at it, improve the function's commentary a bit.

Discussion: https://postgr.es/m/29173.1518282748@sss.pgh.pa.us

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