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