Copy-on-write for record values (#12305)

# Description
This adds a `SharedCow` type as a transparent copy-on-write pointer that
clones to unique on mutate.

As an initial test, the `Record` within `Value::Record` is shared.

There are some pretty big wins for performance. I'll post benchmark
results in a comment. The biggest winner is nested access, as that would
have cloned the records for each cell path follow before and it doesn't
have to anymore.

The reusability of the `SharedCow` type is nice and I think it could be
used to clean up the previous work I did with `Arc` in `EngineState`.
It's meant to be a mostly transparent clone-on-write that just clones on
`.to_mut()` or `.into_owned()` if there are actually multiple
references, but avoids cloning if the reference is unique.

# User-Facing Changes
- `Value::Record` field is a different type (plugin authors)

# Tests + Formatting
- 🟢 `toolkit fmt`
- 🟢 `toolkit clippy`
- 🟢 `toolkit test`
- 🟢 `toolkit test stdlib`

# After Submitting
- [ ] use for `EngineState`
- [ ] use for `Value::List`

crates/nu-cmd-extra/src/extra/filters/roll/mod.rs

@@ -58,7 +58,7 @@ fn horizontal_rotate_value(
         Value::Record { val: record, .. } => {
             let rotations = by.map(|n| n % record.len()).unwrap_or(1);
 
-            let (mut cols, mut vals): (Vec<_>, Vec<_>) = record.into_iter().unzip();
+            let (mut cols, mut vals): (Vec<_>, Vec<_>) = record.into_owned().into_iter().unzip();
             if !cells_only {
                 match direction {
                     HorizontalDirection::Right => cols.rotate_right(rotations),

crates/nu-cmd-extra/src/extra/filters/rotate.rs

@@ -171,7 +171,7 @@ pub fn rotate(
             let span = val.span();
             match val {
                 Value::Record { val: record, .. } => {
-                    let (cols, vals): (Vec<_>, Vec<_>) = record.into_iter().unzip();
+                    let (cols, vals): (Vec<_>, Vec<_>) = record.into_owned().into_iter().unzip();
                     old_column_names = cols;
                     new_values.extend_from_slice(&vals);
                 }

crates/nu-cmd-extra/src/extra/filters/update_cells.rs

@@ -154,7 +154,8 @@ impl Iterator for UpdateCellIterator {
                 let span = val.span();
                 match val {
                     Value::Record { val, .. } => Some(Value::record(
-                        val.into_iter()
+                        val.into_owned()
+                            .into_iter()
                             .map(|(col, val)| match &self.columns {
                                 Some(cols) if !cols.contains(&col) => (col, val),
                                 _ => (