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-command/src/math/utils.rs

@@ -80,15 +80,15 @@ pub fn calculate(
             ),
             _ => mf(vals, span, name),
         },
-        PipelineData::Value(Value::Record { val: record, .. }, ..) => {
-            let mut record = record;
+        PipelineData::Value(Value::Record { val, .. }, ..) => {
+            let mut record = val.into_owned();
             record
                 .iter_mut()
                 .try_for_each(|(_, val)| -> Result<(), ShellError> {
                     *val = mf(slice::from_ref(val), span, name)?;
                     Ok(())
                 })?;
-            Ok(Value::record(*record, span))
+            Ok(Value::record(record, span))
         }
         PipelineData::Value(Value::Range { val, .. }, ..) => {
             let new_vals: Result<Vec<Value>, ShellError> = val