Speed up StateReachability pass for large state machines

[tbennun]

The current algorithm in networkx is suboptimal. With this change, I noticed a 55x speedup in computing state reachability on realistic programs.

[tbennun]

@acalotoiu @mcopik tests fail because of an unrelated issue (has to do with fparser)

[mcopik]

@tbennun Indeed, it looks like an incorrect dependency version. It's really strange because it should have popped up earlier. I will look into this.

[mcopik]

@tbennun @acalotoiu It seems the module structure changed in fparser between 0.1.1 and 0.1.2. In the latter, they introduced a sub-module for Fortran2008, which should work with this import structure. However, our requirements specify exactly the version 0.1.2 - I'm not sure why an older version would be installed.

[mcopik]

@acalotoiu @tbennun Mystery solved - they released a new version two days ago, drastically changing the package structure. While requirements.txt specifies an exact match on 0.1.2, our setup.py has a different dependency: fparser >= 0.1.2.

I think it's better to move forward with the newer version - I will propose a PR to resolve this issue.

[acalotoiu]

LGTM. Thank you! It is a great performance improvement :).