Allow aggregates to provide estimates of their transition state data size.

Formerly the planner had a hard-wired rule of thumb for guessing the amount
of space consumed by an aggregate function's transition state data.  This
estimate is critical to deciding whether it's OK to use hash aggregation,
and in many situations the built-in estimate isn't very good.  This patch
adds a column to pg_aggregate wherein a per-aggregate estimate can be
provided, overriding the planner's default, and infrastructure for setting
the column via CREATE AGGREGATE.

It may be that additional smarts will be required in future, perhaps even
a per-aggregate estimation function.  But this is already a step forward.

This is extracted from a larger patch to improve the performance of numeric
and int8 aggregates.  I (tgl) thought it was worth reviewing and committing
this infrastructure separately.  In this commit, all built-in aggregates
are given aggtransspace = 0, so no behavior should change.

Hadi Moshayedi, reviewed by Pavel Stehule and Tomas Vondra
Improve performance of numeric sum(), avg(), stddev(), variance(), etc.

This patch improves performance of most built-in aggregates that formerly
used a NUMERIC or NUMERIC array as their transition type; this includes
not only aggregates on numeric inputs, but some aggregates on integer
inputs where overflow of an int8 value is a possibility.  The code now
uses a special-purpose data structure to avoid array construction and
deconstruction overhead, as well as packing and unpacking overhead for
numeric values.

These aggregates' transition type is now declared as INTERNAL, since
it doesn't correspond to any SQL data type.  To keep the planner from
thinking that that means a lot of storage will be used, we make use
of the just-added pg_aggregate.aggtransspace feature.  The space estimate
is set to 128 bytes, which is at least in the right ballpark.

Hadi Moshayedi, reviewed by Pavel Stehule and Tomas Vondra
Add make_date() and make_time() functions.

Pavel Stehule, reviewed by Jeevan Chalke and Atri Sharma