Add support for including unordered nodes

[algomaster99]

This feature is useful for nodes in the AST of source code (and even bytecode) that are independent of order. For example, import statements, method declarations. etc.

The change would require:

1. Creating an implementation of Tree interface which can hold the property that the children nodes are unordered.
2. Checks in ChawatheGenerator to eliminate the move actions if the parent of node is UNORDERED and they are mapped to the same parent.