Refactor ExecutingInvocationUnit to use customizable executors

[kilzm]

This refactors the old ExecutingInvocationUnit and sensibly separates its responsibilities into other classes.
It now maintains a configurable list of executors, each of which provides a way to obtain the result of a method within a range of supported methods.
A major advantage of this approach is that executors can handle a method however they want, e.g., avoiding reflection.

The old behavior is kept as a default, meaning a StringReflectionExecutor will be added unless explicitly disabled.

Aside from simplifying major parts of the logic, some improvements to the functionality have been made:

• Support all types of 1D arrays as parameters including arrays of references when using reflection (added tests for String.join() and String.format()).
• An equals check in ExecutingInvocationUnit makes sure that the instance of the method in stack and variables will always be updated if it changed.

[capoz]

Looks good to me, I would just add a changelog entry for stuff you have fixed and other functional changes (e.g., the value analysis fix for category 2 values, the array support to handle String.format and similar methods)