Support time math

[rnett]

There are a number of time math functions, that while supported in Kotlin and commonly in databases (I don't know exactly how prevalent they are, but I suspect very), aren't possible to express in Exposed without resorting to custom functions.

For example <timestamp> + <duration> * <int>.

Currently, arithmetic ops on time types require using the same type, which can get a bit non-sensical.

I think the two primary ops that would be good to have are <timestamp> +|- <duration> and <duration> *|/ <scalar>