leverage `itemize_list` and `write_list` to recover trait bound comments

[ytmimi]

Fixes #2055

Now when using version::Two, rustfmt will permit comments within trait bounds and actually reformat them instead of refusing to rewrite the entire trait definition because there are comments in the bounds.

This follows the approach hinted at in https://github.com/rust-lang/rustfmt/issues/2055#issuecomment-343150579.

Edit -- Other issues fixed by this change: Fixes #6127

[ytmimi]

TODO: add test case for #5321, which I think this also fixes