Replace `CalendarDiffTime` with a more useful type, like `NominalDiffTime`

[sir4ur0n]

Currently Rel8.Expr.Time treats durations (SQL type interval) as CalendarDiffTime.

The problem is that this type is rather useless.
E.g.:

• you can't find the exact number of days this amounts to
• you can't compare values of this type, only test equality
• this type is undocumented

This type's only vaguely useful destructors are its fields ctMonths and ctTime, unfortunately ctMonths representing a number of months, one cannot find the duration it amounts to. CalendarDiffTime {ctMonths = 1, ctTime = fromInteger 0} has a different meaning (hence, duration value, hence number of days) between January and February vs between February and March.

How about switching to a more useful type, e.g. NominalDiffTime?
This type has many useful destructors, like Ord and Num instances (e.g. one can myNominalDiffTime / nominalDay to find the number of days).