Develop rule inlining

[gfngfn]

Note: This PR includes #72. You can see the essential diffs here.

The present PR implements the rule inlining conversion, which transforms

+foo(X) :- bar(X, _).
bar(A, B) :- qux(A, B, _). *)

into

+foo(X) :- qux(X, GenV1, GenV3).
bar(A, B) :- qux(A, B, GenV2).

for instance.

The extended program will newly depend on the ocamlgraph package (version 2.0.0) in order to perform topological sorting.