More custom generators for classes from standard library

[AbdullinAM]

With the introduction of oracles, there is a new problem. ActionSequence generators currently do not adequately support standard library classes, only classes from kex-rt. In these cases our oracles will fail, because expected values will be generated from std classes while arguments will be generated from kex-rt.

To solve that, we need to introduce ways to generate std classes similarly to kex-rt. #112 already implemented some basic classes, e.g.: ArrayList, LinkedList, HashSet and HashMap. But we need to extend that, at least with:

• [ ] IdentityHashMap
• [ ] StringBuilder/StringBuffer
• [ ] TreeSet/TreeMap
• [ ] ...