Implemented Set and Erase operations on SpanTree. Also modified SpanTree, that it have much more Empty nodes, and those empty nodes now are used for inserting.
Note: Because we're approaching deadline, functions added to chains have not unit tests for their own, although they're covered by spantree operations tests.
Original commit: a2701a0cdd
Created a new SpanTree crate in IDE. The structure itself is quite straightforward. To generate span-tree from AST you may use method generate_tree from generator::SpanTreeGenerator trait.
Original commit: 194caafd09
