Implement S2E API so that S2E-based tests can be used with DeepState

[pgoodman]

For example, s2e has the function s2e_make_symbolic, which can be implemented in terms of DeepState_Symbolize.

See more examples here: https://github.com/S2E/docs/blob/master/src/ManualTesting.rst#preparing-the-program-for-s2e