Add slither integration

[anishnaik]

Closes #504
Closes #503
Closes #52

This PR integrates Slither into medusa. In addition to the basic integration where we capture the constants found by Slither, we also implement caching of the Slither results. Thus, when you run medusa the first time on a codebase, the results from slither are cached. These results are then re-used in subsequent fuzzing campaigns.

We do not run slither again for two reasons:

1. Re-running Slither on large projects every single time medusa starts up can save up to many minutes. Using the cache makes the process relatively instantaneous.
2. We are gambling that, if you are fuzzing the codebase, that the underlying system is no longer changing. Thus, the constants remain...constant...