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_plugin_polars/src/dataframe/values/nu_dataframe/mod.rs

@@ -174,9 +174,11 @@ impl NuDataFrame {
 
                     conversion::insert_record(&mut column_values, record, &maybe_schema)?
                 }
-                Value::Record { val: record, .. } => {
-                    conversion::insert_record(&mut column_values, *record, &maybe_schema)?
-                }
+                Value::Record { val: record, .. } => conversion::insert_record(
+                    &mut column_values,
+                    record.into_owned(),
+                    &maybe_schema,
+                )?,
                 _ => {
                     let key = "0".to_string();
                     conversion::insert_value(value, key, &mut column_values, &maybe_schema)?