Switch dependencies from safetimeout to safe-timers

[JamesMGreene]

After wasting a few hours redesigning my old module safetimeout and deciding to rename it, I found that the name I wanted was already taken by a package that has already done a better job of all this than my old module here. 😕

And so, I recommend you switch over to using safe-timers: https://www.npmjs.com/package/safe-timers

P.S. safe-timers does not have 100% API parity, so watch out for that (or encourage them to add it over here).