New lint: simplify duplicate traits in bounds

[scottmcm]

fn foo() -> impl Iterator<Item=i32> + ExactSizeIterator { 0..10 }

can be simplified down to just

fn foo() -> impl ExactSizeIterator<Item=i32> { 0..10 }

[y21]

This lint was added in #11362. Output for the code above:

warning: this bound is already specified as the supertrait of `ExactSizeIterator`
  --> test.rs:22:22
   |
22 |     fn foo() -> impl Iterator<Item = i32> + ExactSizeIterator {
   |                      ^^^^^^^^^^^^^^^^^^^^
help: try removing this bound
   |
22 -     fn foo() -> impl Iterator<Item = i32> + ExactSizeIterator {
22 +     fn foo() -> impl ExactSizeIterator {
   |

[y21]

Closing, since it now exists