Use `EffectFn` instead of unsafe `mkEffect` util

[colinwahl]

I was trying to run some code that used this library through purescript-backend-optimizer when I ran into a runtime error.

For details on the runtime error and what caused it, you can see the discussion in aristanetworks/purescript-backend-optimizer#29.

However, the TLDR of that discussion is that this library should be changed to use EffectFn instead of constructing effects via the unsafe mkEffect util currently defined in Node.FS.Internal (my assumption is that mkEffect predates the existance of EffectFn in the ecosystem).

[colinwahl]

Hah! Just realized this is a duplicated of #28 - my bad.