State the C value feeding a row column two C values of its type fit

[estebanzimanyi]

meta/sql-columns.json states, beside the ordinal and the offset a wrapper computes, the C value
feeding a column where the types leave it open: "from" is "return" or the name of an
out-parameter, and the column takes that value. A statement naming no C value of the function
stops the catalog, as a column nothing feeds does. jsonbEachText states its key from the
returned array and its value from the out-parameter values.

Witness: jsonb_each_text returns its keys and fills its values, both text, and jsonbEachText
returns rows (key text, value text). Over a MobilityDB tree deploying jsonbEachText, run.py
stops with "SQL rows with a column MEOS states no source for ... jsonb_each_text:
jsonbEachText(jsonb) RETURNS record", since both C values fit both columns. With this change
the catalog reads key from return and value from values.

Why: matching a column by type refuses to choose between two C values of one type, and it must,
since the SQL column order and the C parameter order need not agree; the source is then a fact
only the function's author knows, and meta/sql-columns.json is where the catalog reads such
facts. A statement that only skipped its column, as the ordinal does, left both C values unfed.

Measured: over MobilityDB 453039dd6c the catalog run.py derives with this change is
byte-identical to the one it derives without it. Over the MobilityDB branch
meos/host-base-sqlfn it derives, where it stopped before, and jsonbEachText carries the columns
above. tests/test_sqlfn_rows.py states that two C values of one type need a statement, that a
statement feeds its column, and that a statement naming no C value stops the catalog; the suite
passes 487 tests with none skipped, above the floor of 469.