Introduce composable symbolic execution interface in pyx

[Stevengre]

1. provide CSE interface in pyk;
   1. Top-down: The initial state of the specification is state that hits it during its caller's exploration.
   2. Incremental: The summaries are generated when the previous summaries are not launched.
2. Existing Work: CSE in kontrol; jh/ksum-cse-evaluation in mir-semantics;