Try out clippy::unchecked_duration_subtraction

@trinity-1686a writes on irc:

we got a couple issues with that when running tests on some CI that relied on freshly booted VMs (travis?)

I wouldn't be at all surprised to find we had quite a few potential crash bugs, when the user's system has crazy clock behaviour.

We should try building with this lint (and maybe other related ones) and see what comes up. If we like it, we should add it to our standard lint block.