Investigate switching codebase to Hack #94
Description
I wrote a lot of code at Facebook and miss Hack's type checking. This library could very much benefit from using types and is small enough that it could probably be done in one go.
It looks like we could use something like https://docs.hhvm.com/hack/tools/hackificator to convert quickly and something like https://docs.hhvm.com/hack/tools/transpiler to make the library vanilla PHP for consumers.