Support time math

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 ` + * `.

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 ` +|- ` and ` *|/ `