Enable clippy::implied_bounds_in_impls (#8906)

This PR enables the
[`clippy::implied_bounds_in_impls`](https://rust-lang.github.io/rust-clippy/master/index.html#/implied_bounds_in_impls)
rule and fixes the outstanding violations.

Release Notes:

- N/A

Cargo.toml

@@ -375,7 +375,6 @@ borrowed_box = "allow"
 cast_abs_to_unsigned = "allow"
 derive_ord_xor_partial_ord = "allow"
 eq_op = "allow"
-implied_bounds_in_impls = "allow"
 let_underscore_future = "allow"
 map_entry = "allow"
 never_loop = "allow"

crates/workspace/src/pane.rs

@@ -630,7 +630,7 @@ impl Pane {
         self.items.len()
     }
 
-    pub fn items(&self) -> impl Iterator<Item = &Box<dyn ItemHandle>> + DoubleEndedIterator {
+    pub fn items(&self) -> impl DoubleEndedIterator<Item = &Box<dyn ItemHandle>> {
         self.items.iter()
     }