Clippy warning emitted in `DecodeWithMemTracking`.

[gui1117]

This code will trigger a clippy warning:

use parity_scale_codec::{Decode, DecodeWithMemTracking, Encode};

#[derive(Decode, DecodeWithMemTracking, Encode)]
pub struct InnerBound<Balance: Decode>(Balance);

warning: bound is defined in more than one place
 --> src/lib.rs:4:23
  |
4 | pub struct InnerBound<Balance: Decode>(Balance);
  |                       ^^^^^^^          ^^^^^^^
  |
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#multiple_bound_locations
  = note: `#[warn(clippy::multiple_bound_locations)]` on by default

The warning will disappear if we remove DecodeWithMemTracking derivation.