Refactor Digraph to use Data.Graph when possible

Authored by ezyang.

Description

Refactor Digraph to use Data.Graph when possible

Summary: This just rewrites the IntGraph data type.

Signed-off-by: Edward Z. Yang <ezyang@cs.stanford.edu>

Test Plan: validate

Reviewers: austin

Subscribers: thomie

Differential Revision: https://phabricator.haskell.org/D708