Fix clippy lints for rust 1.62 (#3300)

## Issue Addressed

Fixes some new clippy lints after the last rust release
### Lints fixed for the curious:
- [cast_abs_to_unsigned](https://rust-lang.github.io/rust-clippy/master/index.html#cast_abs_to_unsigned)
- [map_identity](https://rust-lang.github.io/rust-clippy/master/index.html#map_identity) 
- [let_unit_value](https://rust-lang.github.io/rust-clippy/master/index.html#let_unit_value)
- [crate_in_macro_def](https://rust-lang.github.io/rust-clippy/master/index.html#crate_in_macro_def) 
- [extra_unused_lifetimes](https://rust-lang.github.io/rust-clippy/master/index.html#extra_unused_lifetimes)
- [format_push_string](https://rust-lang.github.io/rust-clippy/master/index.html#format_push_string)

beacon_node/beacon_chain/src/beacon_chain.rs

@@ -1079,12 +1079,10 @@ impl<T: BeaconChainTypes> BeaconChain<T> {
     }
 
     /// Apply a function to the canonical head without cloning it.
-    pub fn with_head<U, E>(
-        &self,
-        f: impl FnOnce(&BeaconSnapshot<T::EthSpec>) -> Result<U, E>,
-    ) -> Result<U, E>
+    pub fn with_head<U, E, F>(&self, f: F) -> Result<U, E>
     where
         E: From<Error>,
+        F: FnOnce(&BeaconSnapshot<T::EthSpec>) -> Result<U, E>,
     {
         let head_lock = self
             .canonical_head