Refactor Merge Strategy.

[eliyaoo32]

Currently, the merge strategy is mainly doing the String process without encapsulating the entities.
The merge strategies code can be refactored and become more readable.

1. Wrap inside a Blif/Aiger class the logic and conversion.
2. Instead of using CLI tools of aigtoaig, aigtoblif, ABC. We should import them directly to the code.