Create routine able to set single-call SRFs for Materialize mode

Create routine able to set single-call SRFs for Materialize mode

Set-returning functions that use the Materialize mode, creating a
tuplestore to include all the tuples returned in a set rather than doing
so in multiple calls, use roughly the same set of steps to prepare
ReturnSetInfo for this job:
- Check if ReturnSetInfo supports returning a tuplestore and if the
materialize mode is enabled.
- Create a tuplestore for all the tuples part of the returned set in the
per-query memory context, stored in ReturnSetInfo->setResult.
- Build a tuple descriptor mostly from get_call_result_type(), then
stored in ReturnSetInfo->setDesc.  Note that there are some cases where
the SRF's tuple descriptor has to be the one specified by the function
caller.

This refactoring is done so as there are (well, should be) no behavior
changes in any of the in-core functions refactored, and the centralized
function that checks and sets up the function's ReturnSetInfo can be
controlled with a set of bits32 options.  Two of them prove to be
necessary now:
- SRF_SINGLE_USE_EXPECTED to use expectedDesc as tuple descriptor, as
expected by the function's caller.
- SRF_SINGLE_BLESS to validate the tuple descriptor for the SRF.

The same initialization pattern is simplified in 28 places per my
count as of src/backend/, shaving up to ~900 lines of code.  These
mostly come from the removal of the per-query initializations and the
sanity checks now grouped in a single location.  There are more
locations that could be simplified in contrib/, that are left for a
follow-up cleanup.

fcc2817, 07daca5 and d61a361 have prepared the areas of the code related
to this change, to ease this refactoring.

Author: Melanie Plageman, Michael Paquier
Reviewed-by: Álvaro Herrera, Justin Pryzby
Discussion: https://postgr.es/m/CAAKRu_azyd1Z3W_r7Ou4sorTjRCs+PxeHw1CWJeXKofkE6TuZg@mail.gmail.com