Upgrade to lts-15.8

[rossabaker]

• Need to add some MonadFail constraints for ghc-8.8, and an orphan to use it.
• Can't derive ParseTime instances in time-1.9. We get around this by specializing a couple helpers.

[jasonpunzalan]

😷

[andreyk0]

👍 Interesting, I guess that's fine, it's a Date anyway.

[rossabaker]

For posterity, the ParseTime issue. I created one relying on the internals, but even then, one of the two failed the property tests. If we ever really need it, we should look at the time-1.10 upgrade. But I don't anticipate that's a battle we'll ever need to fight.