Allow field name declaration in ROW literal

[dain]

Description

Add support for row(a 1, b 2) instead of the much more complex cast(row(1, 2) as row(a integer, b integer)). The old syntax is particularly annoying because you have to repeat the types for the fields.

Release notes

(X) Release notes are required, with the following suggested text:

## General
* Allow field name declaration in ROW literal.  For example, `row(a 1, b 2)` is now legal. ({issue}`issuenumber`)

[martint]

I like this, but I would use the following syntax to make it similar to how the implicit row in the SELECT clause is constructed:

row(<expr> (AS? <identifier>)?, ...)

That way, the only syntactic difference with the SELECT clause, is that the fields are wrapped in row(...)

[martint]

As another example, the syntax I suggested above makes these two equivalent:

SELECT 1 AS x, 2 AS y

VALUES row(1 AS x, 2 AS y)

[dain]

As another example, the syntax I suggested above makes these two equivalent:

SELECT 1 AS x, 2 AS y

VALUES row(1 AS x, 2 AS y)

I'm working on the syntax change, but for VALUES the names of the relation aren't sourced from the rows. Today you can name row fields with a cast in values, but they don't effect the values column aliases:

values cast(row(1, 2) as row(foo bigint, bar bigint));
 _col0 | _col1 
-------+-------
     1 |     2 
(1 row)

I'm guessing that can be improved, but I have no idea how... work for follow up

[martint]

It shouldn't be necessary to add the field names in the IR. As far as I recall, there's nothing in the planner/optimizer that can or needs to use them.