Use slices directly instead of &Vec (#10328)

Simplifies the signature, makes it more flexible.
Detected a few unnecessary allocations in the process.

crates/nu-cli/src/completions/command_completions.rs

@@ -263,7 +263,7 @@ mod command_completions_tests {
             ("	hello sud", 1),
         ];
         for (idx, ele) in commands.iter().enumerate() {
-            let index = find_non_whitespace_index(&Vec::from(ele.0.as_bytes()), 0);
+            let index = find_non_whitespace_index(ele.0.as_bytes(), 0);
             assert_eq!(index, ele.1, "Failed on index {}", idx);
         }
     }

crates/nu-cli/src/syntax_highlight.rs

@@ -144,7 +144,7 @@ impl Highlighter for NuHighlighter {
 fn split_span_by_highlight_positions(
     line: &str,
     span: Span,
-    highlight_positions: &Vec<usize>,
+    highlight_positions: &[usize],
     global_span_offset: usize,
 ) -> Vec<(Span, bool)> {
     let mut start = span.start;