Improve select row perf for large N (#10355)

# Description
While reviewing #10350 I noticed that a `HashSet<usize>` was used to
deduplicate the incoming rows, which are then sorted after cloning to a
separate `Vec`. This sounds like a candidate for a `BTreeSet` which
guarantees the ordering.

In the process I removed some dead code.

- Use `BTreeSet` instead of `HashSet`
- Remove dead `skip` logic
- Use `BTreeSet` directly in `NthIterator`
- Consume `BTreeSet` through `Peekable<IntoIter>`

crates/nu-command/src/filters/select.rs

@@ -5,7 +5,7 @@ use nu_protocol::{
     Category, Example, IntoInterruptiblePipelineData, IntoPipelineData, PipelineData,
     PipelineIterator, Record, ShellError, Signature, Span, SyntaxShape, Type, Value,
 };
-use std::collections::HashSet;
+use std::collections::BTreeSet;
 
 #[derive(Clone)]
 pub struct Select;
@@ -197,7 +197,7 @@ fn select(
     columns: Vec<CellPath>,
     input: PipelineData,
 ) -> Result<PipelineData, ShellError> {
-    let mut unique_rows: HashSet<usize> = HashSet::new();
+    let mut unique_rows: BTreeSet<usize> = BTreeSet::new();
 
     let mut new_columns = vec![];
 
@@ -224,18 +224,15 @@ fn select(
         };
     }
     let columns = new_columns;
-    let mut unique_rows: Vec<usize> = unique_rows.into_iter().collect();
 
     let input = if !unique_rows.is_empty() {
-        unique_rows.sort_unstable();
         // let skip = call.has_flag("skip");
         let metadata = input.metadata();
         let pipeline_iter: PipelineIterator = input.into_iter();
 
         NthIterator {
             input: pipeline_iter,
-            rows: unique_rows,
-            skip: false,
+            rows: unique_rows.into_iter().peekable(),
             current: 0,
         }
         .into_pipeline_data(engine_state.ctrlc.clone())
@@ -336,8 +333,7 @@ fn select(
 
 struct NthIterator {
     input: PipelineIterator,
-    rows: Vec<usize>,
-    skip: bool,
+    rows: std::iter::Peekable<std::collections::btree_set::IntoIter<usize>>,
     current: usize,
 }
 
@@ -346,32 +342,18 @@ impl Iterator for NthIterator {
 
     fn next(&mut self) -> Option<Self::Item> {
         loop {
-            if !self.skip {
-                if let Some(row) = self.rows.first() {
-                    if self.current == *row {
-                        self.rows.remove(0);
-                        self.current += 1;
-                        return self.input.next();
-                    } else {
-                        self.current += 1;
-                        let _ = self.input.next();
-                        continue;
-                    }
-                } else {
-                    return None;
-                }
-            } else if let Some(row) = self.rows.first() {
+            if let Some(row) = self.rows.peek() {
                 if self.current == *row {
-                    self.rows.remove(0);
+                    self.rows.next();
+                    self.current += 1;
+                    return self.input.next();
+                } else {
                     self.current += 1;
                     let _ = self.input.next();
                     continue;
-                } else {
-                    self.current += 1;
-                    return self.input.next();
                 }
             } else {
-                return self.input.next();
+                return None;
             }
         }
     }