Deref &String arguments to &str where appropriate (#10321)

# Description
This generally makes for nicer APIs, as you are not forced to use an
existing allocation covering the full `String`.

Some exceptions remain where the underlying type requirements favor it.

# User-Facing Changes
None

crates/nu-cli/src/eval_file.rs

@@ -116,9 +116,8 @@ pub fn evaluate_file(
         if block.signature.name == "main" {
             block.signature.name = source_filename.to_string_lossy().to_string();
         } else if block.signature.name.starts_with("main ") {
-            block.signature.name = source_filename.to_string_lossy().to_string()
-                + " "
-                + &String::from_utf8_lossy(&block.signature.name.as_bytes()[5..]);
+            block.signature.name =
+                source_filename.to_string_lossy().to_string() + " " + &block.signature.name[5..];
         }
     }