Name the C value feeding each column of a returned SQL row

[estebanzimanyi]

A SQL signature returning a row lists its columns, each naming the C value
that feeds it. PostgreSQL declares the columns of a function returning
record as its OUT arguments, and those of a function returning a composite
type as the members of that type. An OUT argument is no input, so it leaves
args, argDefaults and required: eDisjointPairs(tgeometry[], tgeometry[]) is
{args: [tgeometry[], tgeometry[]], ret: record, retSet: true, columns:
[i, j]}: the two arrays are its only arguments.

attach_row_sources matches each column to its C source by type, so the SQL
column order and the C parameter order need not agree. from is return, the
value or array the function returns, or an out-parameter, with element or
field inside it: valueTimeSplit rows (number, time, tnumber) read the
out-parameters value_bins and time_bins and the returned fragments,
dynTimeWarpPath rows (i, j) the fields of each returned Match, quaternion
the four elements of the returned array. A bool returned beside
out-parameters says whether there is a row and feeds no column, and a
struct a class stands for, a TBox tile, is one value and not its fields.
meta/sql-columns.json states what only the wrapper computes: the index of a
tile numbers the rows from 1 (from: ordinal), and the …Pairs wrappers add 1
to the C indices i and j to make them SQL array positions (offset: 1). A row
with a column no C value feeds, fed alike by two, or with a C value feeding
no column stops the catalog.

Why. A binding that returns the rows of a SQL function reads them from the
C call: it has to know which C value is which column, and which argument of
a record-returning signature is passed rather than returned.

Measured. Over MobilityDB 8a23781e4c, 147 SQL signatures carried by 64
functions state their columns: 37 returning record (the …Pairs kernels) and
110 returning one of 61 composite types. Of their columns, 200 read the C
return (95 the value or array whole, 74 an element plus the declared
offset, 24 a struct field, 7 an element of a fixed array), 96 an
out-parameter, and 18 the ordinal. The 37
record signatures drop their OUT arguments from args, and the 12 …Pairs
functions their sqlArity from 4, 5 or 6 to 2 or 3. The catalog is otherwise
identical to the one derived without the change.

Witness. tests/test_sqlfn_rows.py takes the OUT arguments out of args and
the members of a composite return type as columns, matches columns by type
whatever their order, reads struct fields, fixed-array elements and a
pair's elements beside an out-parameter array, applies ordinal and offset
only where declared, stops on an unfed, doubly fed or surplus source, and
validates meta/sql-columns.json against its schema. The suite floor goes
from 317 to 337.