Support STP and Yices2

[vcanumalla]

This PR adds support for STP and Yices2 to improve our portfolio solving.

It adds

• Match arm branches of solver flag parsing inmain.rkt that allows the user to pass in STP or Yices2 as the solver.
• Lines in the lakeroad-portfolio.py that add stp and yices2 as options for portfolio solving.

Note that this PR doesn't add functionality for passing flags/solver settings to STP or Yices2; we haven't needed them yet.

Closes #406

[gussmith23]

Great work. There's one more thing that needs to be done, and that's getting STP support into Rosette. We have two options here:

1. Open a PR on Rosette and get it merged before merging this PR, or
2. Add modified Rosette (with STP support) as a submodule to Lakeroad. This would also require updating the Dockerfile so that it installs Rosette from the submodule.

Given that we want to get STP and Yices2 support into Rosette anyway, I say we take option 1. This means that you should prioritize opening up the STP and Yices2 PR on Rosette ASAP so that we can get their CI working. I suspect they'll be happy to merge after that. This PR will be blocked until they merge.

This looks really good! I especially appreciate the detailed PR message.

[gussmith23]

Had to manually rerun tests and force total rebuild so that new Rosette got pulled. That manual rerun passed, so merging now.