Closes #64.

Implemented a new class Assumption that contains required constraints for the symbolic execution, but which not nessesarily should be satisfied in real (concrete) execution.

Now UtSolver has three types of constraints:

1. Hard constraints -- they must be satisfied at any point of the analysis;
2. Soft constraints -- the solver tries to satisfy them, but they will be dropped if it is impossible or we couldn't determine if it possible during the timeout
3. Assumptions -- they must be satisfied during the symbolic execution, but if at some point of the analysis we will receive unsat status from the solver, one of the assumptions will be in the produced unsat core and without it the request to the solver is SAT, we will run concrete execution from that point.

There were added corresponding tests for it and changes in utbot-api module.

Note: I've tried to remove all the assume instructions from the preconditionChecks of the wrappers, but seems like it is impossible because of untimely contradiction raised in the preconditionCheck analysis of the UtHashMap and UtHashSet (in the cycle). More details you can find in the design doc docs/AssumptionsMechanism.md