Changed SRF in targetlist handling

Hi,

discussing executor performance with a number of people at pgcon,
several hackers - me included - complained about the additional
complexity, both code and runtime, required to handle SRFs in the target
list.

One idea I circulated was to fix that by interjecting a special executor
node to process SRF containing targetlists (reusing Result possibly?).
That'd allow to remove the isDone argument from ExecEval*/ExecProject*
and get rid of ps_TupFromTlist which is fairly ugly.

Robert suggested - IIRC mentioning previous on-list discussion - to
instead rewrite targetlist SRFs into lateral joins. My gut feeling is
that that'd be a larger undertaking, with significant semantics changes.

If we accept bigger semantical changes, I'm inclined to instead just get
rid of targetlist SRFs in total; they're really weird and not needed
anymore.

One issue with removing targetlist SRFs is that they're currently
considerably faster than SRFs in FROM:
tpch[14693][1]=# COPY (SELECT * FROM generate_series(1, 10000000)) TO '/dev/null';
COPY 10000000
Time: 2217.167 ms
tpch[14693][1]=# COPY (SELECT generate_series(1, 10000000)) TO '/dev/null';
COPY 10000000
Time: 1355.929 ms
tpch[14693][1]=#

I'm no tto concerned about that, and we could probably fixing by
removing forced materialization from the relevant code path.

Comments?

Greetings,

Andres Freund