Add some infrastructure (mostly macros) to make it easier to write
typical cases for constant-expression simplification.  Add simplification
processing for ArrayRef, RowExpr, and ScalarArrayOpExpr node types,
which formerly went unsimplified even if all their inputs were constants.
Also teach it to simplify FieldSelect from a composite constant.
Make use of the new infrastructure to reduce the amount of code needed
for the existing ArrayExpr and ArrayCoerceExpr cases.

One existing test case changes output as a result of the fact that
RowExpr can now be folded to a constant.  All the new code is exercised
by existing test cases according to gcov, so I feel no need to add
additional tests.

Tom Lane, reviewed by Dmitry Dolgov

Discussion: https://postgr.es/m/3be3b82c-e29c-b674-2163-bf47d98817b1@iki.fi