This PR implements the INTERVAL type that can be used to perform offsets on dates. Internally the interval is represented as follows:

struct interval_t {
	int32_t months;
	int32_t days;
	int64_t msecs;
};

Intervals can be constructed using either the postfix syntax:

SELECT INTERVAL '1' MONTH;
SELECT INTERVAL '1' DAY;

Or using a string:

SELECT INTERVAL '1 month 1 day';

Intervals can be used to do date math:

SELECT DATE '1992-01-01' + INTERVAL '1' MONTH;
-- 1992-02-01

Or timestamp math:

SELECT TIMESTAMP '1992-01-01 01:00:00' + INTERVAL '1 month 1 day 30 minutes 20 seconds';
-- 1992-02-02 01:30:20

Intervals can also be added to each other, or subtracted from each other.

SELECT INTERVAL '1' DAY + INTERVAL '1' SECOND;
-- 1 day 00:00:01

Intervals are now also created by the age function, or when subtracting two timestamps from each other.