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-explore/src/nu_common/table.rs

@@ -16,7 +16,7 @@ pub fn try_build_table(
     let span = value.span();
     match value {
         Value::List { vals, .. } => try_build_list(vals, ctrlc, config, span, style_computer),
-        Value::Record { val, .. } => try_build_map(*val, span, style_computer, ctrlc, config),
+        Value::Record { val, .. } => try_build_map(&val, span, style_computer, ctrlc, config),
         val if matches!(val, Value::String { .. }) => {
             nu_value_to_string_clean(&val, config, style_computer).0
         }
@@ -25,7 +25,7 @@ pub fn try_build_table(
 }
 
 fn try_build_map(
-    record: Record,
+    record: &Record,
     span: Span,
     style_computer: &StyleComputer,
     ctrlc: Option<Arc<AtomicBool>>,
@@ -42,11 +42,11 @@ fn try_build_map(
         0,
         false,
     );
-    let result = ExpandedTable::new(None, false, String::new()).build_map(&record, opts);
+    let result = ExpandedTable::new(None, false, String::new()).build_map(record, opts);
     match result {
         Ok(Some(result)) => result,
         Ok(None) | Err(_) => {
-            nu_value_to_string(&Value::record(record, span), config, style_computer).0
+            nu_value_to_string(&Value::record(record.clone(), span), config, style_computer).0
         }
     }
 }

crates/nu-explore/src/nu_common/value.rs

@@ -87,7 +87,7 @@ pub fn collect_input(value: Value) -> (Vec<String>, Vec<Vec<Value>>) {
     let span = value.span();
     match value {
         Value::Record { val: record, .. } => {
-            let (key, val) = record.into_iter().unzip();
+            let (key, val) = record.into_owned().into_iter().unzip();
             (key, vec![val])
         }
         Value::List { vals, .. } => {

crates/nu-explore/src/pager/mod.rs

@@ -1038,7 +1038,7 @@ fn set_config(hm: &mut HashMap<String, Value>, path: &[&str], value: Value) -> b
             if path.len() == 2 {
                 let key = path[1];
 
-                record.insert(key, value);
+                record.to_mut().insert(key, value);
             } else {
                 let mut hm2: HashMap<String, Value> = HashMap::new();
                 for (k, v) in record.iter() {