Fix clippy warnings (upcoming) (#9282)

# Description

Fixes the clippy warnings we're about to get hit with next time we
upgrade Rust.

The big one was shrinking ShellError and related under 128 bytes.

# User-Facing Changes

Shouldn't notice much difference. In theory, we could see a tiny perf
improvement, but I didn't notice one.

# Tests + Formatting
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crates/nu-protocol/src/engine/engine_state.rs

@@ -2243,7 +2243,7 @@ mod engine_state_tests {
         let variables = working_set
             .list_variables()
             .into_iter()
-            .map(|v| from_utf8(v))
+            .map(from_utf8)
             .collect::<Result<Vec<&str>, Utf8Error>>()?;
         assert_eq!(variables, vec![varname_with_sigil]);
         Ok(())