Add `Time::Span` for `interval` column

[jwoertink]

in postgres, you can do this

# select interval '1 hour';
 interval 
----------
 01:00:00
(1 row)

and in Crystal 1.hour gives you 01:00:00.

[jwoertink]

Got a little too excited, and forgot that 1.month and 1.year return Time::MonthSpan, but postgres 1 month and 1 year are still interval columns... So that would probably be hard to implement :thinking:

[jwoertink]

Related: https://github.com/will/crystal-pg/issues/272