Add Record::remove/retain/retain_mut (#10876)

# Description
While we have now a few ways to add items or iterate over the
collection, we don't have a way to cleanly remove items from `Record`.

This PR fixes that:

- Add `Record.remove()` to remove by key
- makes the assumption that keys are unique, so can not be used
universally, yet (see #10875 for an important example)
- Add naive `Record.retain()` for inplace removal
- This follows the two separate `retain`/`retain_mut` in the Rust std
library types, compared to the value-mutating `retain` in `indexmap`
- Add `Record.retain_mut()` for one-pass pruning

Continuation of #10841 

# User-Facing Changes
None yet.

# Tests + Formatting
Doctests for the `retain`ing fun

crates/nu-protocol/src/value/record.rs

@@ -94,6 +94,104 @@ impl Record {
         Some((self.cols.get(idx)?, self.vals.get(idx)?))
     }
 
+    /// Remove single value by key
+    ///
+    /// Returns `None` if key not found
+    ///
+    /// Note: makes strong assumption that keys are unique
+    pub fn remove(&mut self, col: impl AsRef<str>) -> Option<Value> {
+        let idx = self.index_of(col)?;
+        self.cols.remove(idx);
+        Some(self.vals.remove(idx))
+    }
+
+    /// Remove elements in-place that do not satisfy `keep`
+    ///
+    /// Note: Panics if `vals.len() > cols.len()`
+    /// ```rust
+    /// use nu_protocol::{record, Value};
+    ///
+    /// let mut rec = record!(
+    ///     "a" => Value::test_nothing(),
+    ///     "b" => Value::test_int(42),
+    ///     "c" => Value::test_nothing(),
+    ///     "d" => Value::test_int(42),
+    ///     );
+    /// rec.retain(|_k, val| !val.is_nothing());
+    /// let mut iter_rec = rec.columns();
+    /// assert_eq!(iter_rec.next().map(String::as_str), Some("b"));
+    /// assert_eq!(iter_rec.next().map(String::as_str), Some("d"));
+    /// assert_eq!(iter_rec.next(), None);
+    /// ```
+    pub fn retain<F>(&mut self, mut keep: F)
+    where
+        F: FnMut(&str, &Value) -> bool,
+    {
+        self.retain_mut(|k, v| keep(k, v));
+    }
+
+    /// Remove elements in-place that do not satisfy `keep` while allowing mutation of the value.
+    ///
+    /// This can for example be used to recursively prune nested records.
+    ///
+    /// Note: Panics if `vals.len() > cols.len()`
+    /// ```rust
+    /// use nu_protocol::{record, Record, Value};
+    ///
+    /// fn remove_foo_recursively(val: &mut Value) {
+    ///     if let Value::Record {val, ..} = val {
+    ///         val.retain_mut(keep_non_foo);
+    ///     }
+    /// }
+    ///
+    /// fn keep_non_foo(k: &str, v: &mut Value) -> bool {
+    ///     if k == "foo" {
+    ///         return false;
+    ///     }
+    ///     remove_foo_recursively(v);
+    ///     true
+    /// }
+    ///
+    /// let mut test = Value::test_record(record!(
+    ///     "foo" => Value::test_nothing(),
+    ///     "bar" => Value::test_record(record!(
+    ///         "foo" => Value::test_nothing(),
+    ///         "baz" => Value::test_nothing(),
+    ///         ))
+    ///     ));
+    ///
+    /// remove_foo_recursively(&mut test);
+    /// let expected = Value::test_record(record!(
+    ///     "bar" => Value::test_record(record!(
+    ///         "baz" => Value::test_nothing(),
+    ///         ))
+    ///     ));
+    /// assert_eq!(test, expected);
+    /// ```
+    pub fn retain_mut<F>(&mut self, mut keep: F)
+    where
+        F: FnMut(&str, &mut Value) -> bool,
+    {
+        let mut idx = 0;
+
+        // `Vec::retain` is able to optimize memcopies internally. For maximum benefit as `Value`
+        // is a larger struct than `String` use `retain` on `vals`
+        //
+        // The calls to `Vec::remove` are suboptimal as they need memcopies to shift each time.
+        //
+        // As the operations should remain inplace, we don't allocate a separate index `Vec` which
+        // could be used to avoid the repeated shifting of `Vec::remove` in cols.
+        self.vals.retain_mut(|val| {
+            if keep(self.cols[idx].as_str(), val) {
+                idx += 1;
+                true
+            } else {
+                self.cols.remove(idx);
+                false
+            }
+        });
+    }
+
     pub fn columns(&self) -> Columns {
         Columns {
             iter: self.cols.iter(),