Better coarsetime strategy needed

I was writing some channel padding computations and missed .checked* functions on the types in coarsetime. I investigated a bit and I discovered that the crate is full of unchecked arithmetic on u64. I think these are silent wrapping overflow in release builds, and panics in debug builds. There are none of the customary Panics sections in the docs.

Also, there are no impls of the obvious conversions to or from std::time.

I don't consider myself impressed. We should decide what to do about this.