Evaluate arguments of correlated SubPlans in the referencing ExprState

Until now we generated an ExprState for each parameter to a SubPlan and
evaluated them one-by-one ExecScanSubPlan. That's sub-optimal as creating lots
of small ExprStates
a) makes JIT compilation more expensive
b) wastes memory
c) is a bit slower to execute

This commit arranges to evaluate parameters to a SubPlan as part of the
ExprState referencing a SubPlan, using the new EEOP_PARAM_SET expression
step. We emit one EEOP_PARAM_SET for each argument to a subplan, just before
the EEOP_SUBPLAN step.

It likely is worth using EEOP_PARAM_SET in other places as well, e.g. for
SubPlan outputs, nestloop parameters and - more ambitiously - to get rid of
ExprContext->domainValue/caseValue/ecxt_agg*.  But that's for later.

Author: Andres Freund <andres@anarazel.de>
Reviewed-by: Tom Lane <tgl@sss.pgh.pa.us>
Reviewed-by: Alena Rybakina <lena.ribackina@yandex.ru>
Discussion: https://postgr.es/m/20230225214401.346ancgjqc3zmvek@awork3.anarazel.de

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