Add Access like API for Elixir AST

[doorgan]

Functions like put_in or update_in are very useful to work with nested data, so naturally they'd be nice for Elixir AST.
Unfortunately, the Elixir AST is just tuples, and we can't implement Acesss for them.

Moreover, a hypothetical lens-like operation like update_in(ast, [:def, :function_name], &change_node_name/1) would crash with the Access semantic, as it would first find the def nodes, then crash if any of them doesn't have a function_name child.
So, our implementation needs to ignore the elements it can't match instead of crashing.