Reducing output size of nodeToString

Hi,

PFA a patch that reduces the output size of nodeToString by 50%+ in
most cases (measured on pg_rewrite), which on my system reduces the
total size of pg_rewrite by 33% to 472KiB. This does keep the textual
pg_node_tree format alive, but reduces its size signficantly.

The basic techniques used are
 - Don't emit scalar fields when they contain a default value, and
make the reading code aware of this.
 - Reasonable defaults are set for most datatypes, and overrides can
be added with new pg_node_attr() attributes. No introspection into
non-null Node/Array/etc. is being done though.
 - Reset more fields to their default values before storing the values.
 - Don't write trailing 0s in outDatum calls for by-ref types. This
saves many bytes for Name fields, but also some other pre-existing
entry points.

Future work will probably have to be on a significantly different
storage format, as the textual format is about to hit its entropy
limits.

See also [0], [1] and [2], where complaints about the verbosity of
nodeToString were vocalized.

Kind regards,

Matthias van de Meent

[0] https://www.postgresql.org/message-id/CAEze2WgGexDM63dOvndLdAWwA6uSmSsc97jmrCuNmrF1JEDK7w%40mail.gmail.com
[1] https://www.postgresql.org/message-id/flat/CACxu%3DvL_SD%3DWJiFSJyyBuZAp_2v_XBqb1x9JBiqz52a_g9z3jA%40mail.gmail.com
[2] https://www.postgresql.org/message-id/4b27fc50-8cd6-46f5-ab20-88dbaadca645%40eisentraut.org