Fix the signature of bi-abduction functions

[giltho]

get_fix should return a list of assertion (potential fixes) and apply_fix should disappear, instead the assertion should be produced.
This also allows more powerful fixes like ones containing predicates!

[giltho]

Right now, Asrts are passed from the SState to the PState, and they're applied at the PState level. I don't think any memory model leverages the idea of having something in the Asrt fixes, so we should just get rid of that