GoTo definition for Ast.Variable + Data structure Node[T] for tree traversal

[simerplaha]

• Towards Implement go-to definition #105.
• Implements initial go-to definition support for Ast.Variable.
• Implements building a traversable tree from Ast.ContractWithState.
  • As more AST types implement the Positioned trait, those would automatically get added to this tree, making them available for go-to definitions.
• Implements Node[T] type and functions for tree traversal walkUp, walkDown etc.
• Builds the foundation for supporting go-to definition for other types. It should be quicker to implement them following these functional tree traversals.

goto_def_2.mp4

[simerplaha]

Will reopen.