cleanup execTuples.c

This patch refactors execTuples.c in two ways:

     (1) ExecInitXXXResultTupleSlot() used a macro to avoid some
         duplicated code, whereas calling ExecInitExtraTupleSlot() would
         make the code more clear.

     (2) ExecTypeFromTL() and ExecCleanTypeFromTL() duplicated a bunch
         of code; I added a new function ExecTypeFromTLInternal() and
         re-implemented these functions in terms of calls to it.

As a result, ExecInitScanTupleSlot(), ExecInitResultTupleSlot(),
ExecTypeFromTL(), and ExecCleanTypeFromTL() are now all trivial
(1 line) functions. I could have replaced these with macros, but I
didn't: does anyone thinks that would be worth doing?

-Neil