Capabilities for fast prototyping

[ArmborstL]

It is not nice that VerCors takes a long time every time you make small changes, e.g. when developing specifications. Having a fast mode for prototyping would be nice. This could potentially be unsound by e.g. making assumptions about other parts of the program (a full, slow run of VerCors would be needed after the development is done). This could involve:

• more fine-grained caching (right now only done on method level, but maybe also within the method, e.g. branch-level or at a user-defined caching point in the program). This might require caching at various levels, e.g. in z3 as well as VerCors.
• program slicing: only look at specific branches, ignoring other branches