Implement Default trait for Arena type

[lo48576]

Default is implemented manually for Arena, but not using #[derive(Default)].
This is because #[derive(Default)] adds unnecessary trait bound T: Default automatically (see rust-lang/rust#26925).

Additionally, this change suppresses clippy lint to recommend deriving Default for simple new() method.
Clippy complains about that for previous code:

$ cargo clippy
warning: you should consider deriving a `Default` implementation for `arena::Arena<T>`
  --> src/arena.rs:41:5
   |
41 | /     pub fn new() -> Arena<T> {
42 | |         Arena { nodes: Vec::new() }
43 | |     }
   | |_____^
   |
   = note: #[warn(clippy::new_without_default)] on by default
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#new_without_default
help: try this
   |
35 | #[derive(Default)]
   |

    Finished dev [unoptimized + debuginfo] target(s) in 0.27s
$

[saschagrunert]

LGTM, thanks!