CGO 2020 Artifact Evaluation Release

This is a release for CGO 2020 Artifact Evaluation (instruction is here https://github.com/GraphIt-DSL/graphit/tree/master/graphit_eval/priority_graph_cgo2020_eval).
We have added a large number of features since the last release (Python Binding, Support for PriorityQueue, Intersection Operators, ... ).

Post OOPSLA Release

(1) Bug fixes and making the CMAKE scripts more robust in different environments.
(2) New features for memory deallocation.
(3) Updates to the high-level scheduling APIs and scripts to make it easier to benchmark the performance of GraphIt.

Initial release to make the artifact available for OOPSLA18

Initial release to make the artifact available for OOPSLA18.