Closes #18, closes #32

To increase the performance of symbolic analysis for source code that uses enums, we can extract these enums concretely from runtime instead of enum's initializers analysis which can require a lot of time for enums with a lot of constants (because enum's static initializer fills fields for every enum constant, creates $VALUES array, invokes java.lang.Enum initializer for every constant, etc).

But very important thing is that we cannot process concretely every enum but only isolated - enums that do not affect the system state (like other classes' statics) in their initializers and do not use external statics to initialize its own statics. To prevent concrete processing of enum that is not isolated, we need to traverse its static initializer and check that it does not initialize external statics and does not use external statics to initialize its own statics.