BackwardSlice: some refactoring and interface changes. A few bug fixes.
BackwardSlice: some refactoring and interface changes. A few bug fixes.

- Deprecate "cfg_node" and "stmt_id", and replace them with a "targets" argument. Now BackwardSlice takes multiple targets.
- Bug fix: call to all_simple_paths is very slow on non-trivial CFGs.
- Bug fix: the very last CFGNode in the graph was always ignored.

f-prettyland/angr,schieb/angr,axt/angr,chubbymaggie/angr,schieb/angr,iamahuman/angr,tyb0807/angr,haylesr/angr,angr/angr,tyb0807/angr,chubbymaggie/angr,f-prettyland/angr,tyb0807/angr,angr/angr,iamahuman/angr,schieb/angr,axt/angr,chubbymaggie/angr,angr/angr,axt/angr,f-prettyland/angr,iamahuman/angr,haylesr/angr