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-protocol/src/eval_base.rs

@@ -75,7 +75,7 @@ pub trait Eval {
                         RecordItem::Spread(_, inner) => {
                             match Self::eval::<D>(state, mut_state, inner)? {
                                 Value::Record { val: inner_val, .. } => {
-                                    for (col_name, val) in *inner_val {
+                                    for (col_name, val) in inner_val.into_owned() {
                                         if let Some(orig_span) = col_names.get(&col_name) {
                                             return Err(ShellError::ColumnDefinedTwice {
                                                 col_name,