Improving EXPLAIN's display of SubPlan nodes

EXPLAIN has always been really poor at displaying SubPlan nodes
in expressions: you don't get much more than "(SubPlan N)".
This is mostly because every time I thought about it, I despaired
of trying to represent all the information in a SubPlan accurately.
However, a recent discussion[1] made me realize that we could do
a lot better just by displaying the SubLinkType and the testexpr
(if relevant). So here's a proposed patch. You can see what
it does by examining the regression test changes.

There's plenty of room to bikeshed about exactly how to display
this stuff, and I'm open to suggestions.

BTW, I was somewhat depressed to discover that we have exactly
zero regression coverage of the ROWCOMPARE_SUBLINK code paths;
not only was EXPLAIN output not covered, but the planner and
executor too. So I added some simple tests for that. Otherwise
I think existing coverage is enough for this.

regards, tom lane

[1] https://www.postgresql.org/message-id/flat/149c5c2f-4267-44e3-a177-d1fd24c53f6d%40universite-paris-saclay.fr