Fix clippy (#15489)

# Description
There are some clippy(version 0.1.86) errors on nushell repo. This pr is
trying to fix it.

# User-Facing Changes
Hopefully none.

# Tests + Formatting
NaN

# After Submitting
NaN

crates/nu-protocol/src/engine/state_working_set.rs

@@ -884,7 +884,7 @@ impl<'a> StateWorkingSet<'a> {
                 .active_overlay_names(&mut removed_overlays)
                 .iter()
                 .rev()
-                .last()
+                .next_back()
             {
                 return last_name;
             }
@@ -900,7 +900,7 @@ impl<'a> StateWorkingSet<'a> {
             if let Some(last_overlay) = scope_frame
                 .active_overlays(&mut removed_overlays)
                 .rev()
-                .last()
+                .next_back()
             {
                 return last_overlay;
             }

crates/nu-protocol/src/lev_distance.rs

@@ -88,7 +88,7 @@ pub fn lev_distance_with_substrings(a: &str, b: &str, limit: usize) -> Option<us
         1 // Exact substring match, but not a total word match so return non-zero
     } else if !big_len_diff {
         // Not a big difference in length, discount cost of length difference
-        score + (len_diff + 1) / 2
+        score + len_diff.div_ceil(2)
     } else {
         // A big difference in length, add back the difference in length to the score
         score + len_diff

crates/nu-protocol/src/value/from_value.rs

@@ -133,7 +133,7 @@ pub trait FromValue: Sized {
         Type::Custom(
             any::type_name::<Self>()
                 .split(':')
-                .last()
+                .next_back()
                 .expect("str::split returns an iterator with at least one element")
                 .to_string()
                 .into_boxed_str(),