Implement COW semantics for state objects

[yan]

We currently rely on deep-copying when generating derived states, and use ad-hoc context managers to maintain them. We should implement COW semantics for all shared state data structures.

(This issue is a superset of #166)

[yan]

From earlier issue, describing enter/exit protocol updates:

All objects in State need to implement a temporary copy on write mechanism over we could do a fork.

current_state = State()
with current_state as new_state:
#new_state here most be a COW copy of current state.
#every action over new_state must not affect current_state
#current_state must be inmutable during this with-statement
#after we do operations over new_state we can save it or forget it.
#As after the with statement new_state disappears then current state can be mutable again

Currently this is only implemented for the ConstraintSet and State (not all the objects used in State)

[ehennenfent]

https://github.com/trailofbits/manticore/wiki/Ideas