Support ANSI SQL datetime / interval literals

[nickolay]

See:

• <datetime literal> -- DATE 'yyyy-mm-dd' and similar
• <interval literal> -- INTERVAL '5' DAY
• <datetime value expression>.

An open question is whether the date/time values should store the string values intact or parse the strings and store them as chrono values as in the code I removed in https://github.com/andygrove/sqlparser-rs/pull/61 (andygrove@de177f1) - I'm leaning towards the former, so that a user of this library can choose the preferred representation.

[nickolay]

Some notes on INTERVALs in SQL:

While reading through the grammar rules from the spec and the explanation, I made this cheatsheet for myself:

<interval literal> ::= INTERVAL [ <sign> ] '<value>' <interval qualifier>
<interval type> ::= INTERVAL <interval qualifier>

<interval qualifier> ::=
    SECOND ( <leading precision>, <fractional seconds precision> )
  | <start field> [ ( <leading field precision> ) ]
      [ TO <end field> [ ( <fractional seconds precision> ) ] ]

where <start field> is one of YEAR/MONTH/DAY/HOUR/MINUTE and <end field> is either one of those as well, or SECOND. <start field> is supposed to be a unit that's not smaller than <end field> (although Postgres forbids YEAR TO YEAR in my testing.)

The <start field> and <end field> define the fields expected in the <value>, which follows the format:

• 'yy-mm' for YEAR TO MONTH intervals (or simply 'yy' for YEAR and 'mm' for MONTH intervals)
• 'dd hh:mi:ss.nn' for DAY TO SECOND, or the corresponding subset of that for DAY TO HOUR/MINUTE, HOUR TO MINUTE/SECOND, MINUTE TO SECOND, and DAY/HOUR/MINUTE/SECOND intervals.

The plus/minus sign is also allowed inside the quoted <value>.

(Posgres allows omitting <interval qualifier> in exchange for using more complex value string: INTERVAL '2 years 13 months'.)

The <leading field precision> is the number of digits allowed in the "start field" of the interval, the <fractional seconds precision> is the number of digits allowed in the fractional part the "end field" (but only if the end field is seconds). There's no need to specify precision for other fields, as you're not allowed to write something like '0:120' HOUR TO MINUTE or '33.5' MINUTE (while you can write '1000:0' HOUR TO MINUTE).

[nickolay]

Thanks to all the hard work by @benesch we now have support for DATE/TIME/INTERVAL literals, with the following caveats:

• We don't validate the <interval qualifier> and don't attempt to parse the <value> of INTERVAL/DATE/TIME/TIMESTAMP
• We don't yet support <interval qualifier> after the INTERVAL type
• We don't support the optional sign in INTERVAL [<sign>] '<value>' .. (not sure which DBs support it)
• We don't support PostgreSQL's extension: INTERVAL '<value>' without qualifiers