expr: complete string <-> datum casts

[benesch]

For every datum variant, there should be a cast from string and a cast to string. Once #1377 is complete, the cast functions will be a trivial wrapper over the conversion functions in repr.

[cuongdo]

This is especially important for file-based sources that are parsed using a regex. Each column in such a source is a string, so it's impossible to perform basic calculations such as avg without being able to cast the column into a numeric type.

[benesch]

Hopefully landing this week! The number of roadblocks here has been astounding. #1448 + #1444 + #1429 + some code movement of our existing date/timestamp[tz]/interval logic will finally get us there.

[benesch]

Still remaining:

• text -> date
• text -> timestamp
• text -> timestamptz
• text -> interval

[cuongdo]

Are text -> date casts supposed to work right now? I can't get them to work:

cdo=> SELECT COALESCE(CAST('2020-01-01' AS DATE), '(null)');
 coalesce
----------
 (null)
(1 row)

[benesch]

D'oh, I wrote those all backwards. Sorry.

[benesch]

(As in, text -> {date, timestamp, timestamptz, interval} are all still TODO, because they'll conflict badly with #1444.)

[cuongdo]

Assigning to @sploiselle, who I hear is going to be finishing off the remaining casts as a follow-up to his INTERVAL work