Call DAG generation tool

[kennyzzhang]

Implements a new computation dag generation tool.

When compiling with -fcilktool=cilkgraph and running the executable, the call dag will be output in DOT format as an extra line to stdout.

[behoppe]

Hi @kennyzzhang and thanks for this PR. I see you are a first-time contributor -- welcome! I am not one of the technical experts (like @neboat) who will look at your PR, but I work with the OpenCilk team to help engage our users. So I'm curious if you'd like to say more about what you're doing with OpenCilk and this new computation dag generation tool. Thank you.

[kennyzzhang]

Hey @behoppe, I'm @neboat's new grad student. We're developing this tool to help visualize parallelism within programs, and hopefully be able to see things like which groups of tasks were executed by which worker, when do steals occur and how much communication do they incur, and maybe also log timing information to get a better understanding of what's going on behind the scenes with the scheduler.

It's also an initial project to get me accquainted with the internals of OpenCilk, so that when the time comes for me to experiment with algorithmic changes or implementing features as part of my research, I'll know where in the codebase to look.

[behoppe]

Hi @kennyzzhang, it sounds like you are in a good place :-)
Thanks for your work! I look forward to seeing some visualizations.