Fix build on GHC 8.10

[ocharles]

I believe these changes are also compatible with older GHCs.

Fixes #7

[cdepillabout]

As far as I can tell, this no longer builds on GHC-8.2 or older:

https://travis-ci.org/github/cdepillabout/world-peace/builds/699619671

I'll just disable those builds on CI, and if someone wants support for older GHCs, I'd accept a PR getting that working.

[cdepillabout]

@ocharles Are you guys using world-peace? I depend on it in servant-checked-exceptions, but I wasn't aware of anyone directly using it.

[ocharles]

@cdepillabout yea, we also use that library (GHC 8.8 and above)

[cdepillabout]

@ocharles Oh, nice.

Well if there are any features you guys need, please feel free to open up issues or PRs.

I've basically added everything I needed for servant-checked-exceptions, but I'm not actively working on adding stuff to this library.

Also, if world-peace ever becomes a bottleneck for you, you may be interested in switching to a library that has had more optimizations done.

[ocharles]

Will do, thanks!