Revert "Refactor Digraph to use Data.Graph when possible"

Authored by ezyang.

Description

Revert "Refactor Digraph to use Data.Graph when possible"

This breaks the build with GHC 7.6 bootstrapping, since the Functor SCC
instance is not available.

This reverts commit c439af5f5baa2c8af3434652554135230edbf5c3.