Add implementation of the LEAST/GREATEST functions

[Mytherin]

This PR adds the LEAST/GREATEST functions (#697), examples:

SELECT LEAST(1, 3); -- 1
SELECT LEAST(1, 3, 0); -- 0
SELECT LEAST(1, 3, 0, NULL); -- NULL

LEAST/GREATEST are varargs functions, meaning they accept any number of parameters.

The MySQL variant is implemented, which means that any NULL present results in a NULL as output. The PostgreSQL variant has different behavior here - it ignores any NULL values (similar to CONCAT). I have no strong opinion either way, the MySQL one made a bit more sense to me, but we can change this around if preferred.

The implementation was a bit less straightforward than normal as the Postgres parser had the LEAST/GREATEST hardcoded in rather than as a normal function - no clue why, I have removed this and turned it into a normal function.