Use Record's public API in a bunch of places (#10927)

# Description
Since #10841 the goal is to remove the implementation details of
`Record` outside of core operations.

To this end use Record iterators and map-like accessors in a bunch of
places. In this PR I try to collect the boring cases where I don't
expect any dramatic performance impacts or don't have doubts about the
correctness afterwards

- Use checked record construction in `nu_plugin_example`
- Use `Record::into_iter` in `columns`
- Use `Record` iterators in `headers` cmd
- Use explicit record iterators in `split-by`
- Use `Record::into_iter` in variable completions
- Use `Record::values` iterator in `into sqlite`
- Use `Record::iter_mut` for-loop in `default`
- Change `nu_engine::nonexistent_column` to use iterator
- Use `Record::columns` iter in `nu-cmd-base`
- Use `Record::get_index` in `nu-command/network/http`
- Use `Record.insert()` in `merge`
- Refactor `move` to use encapsulated record API
- Use `Record.insert()` in `explore`
- Use proper `Record` API in `explore`
- Remove defensiveness around record in `explore`
- Use encapsulated record API in more `nu-command`s

# User-Facing Changes
None intentional

# Tests + Formatting
(-)

crates/nu-explore/src/commands/help.rs

@@ -165,7 +165,7 @@ fn help_frame_data(
 
             let (cols, mut vals) = help_manual_data(manual, aliases);
             let vals = vals.remove(0);
-            Value::record(Record { cols, vals }, NuSpan::unknown())
+            Value::record(Record::from_raw_cols_vals(cols, vals), NuSpan::unknown())
         })
         .collect();
     let commands = Value::list(commands, NuSpan::unknown());

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

@@ -90,7 +90,10 @@ fn collect_external_stream(
 pub fn collect_input(value: Value) -> (Vec<String>, Vec<Vec<Value>>) {
     let span = value.span();
     match value {
-        Value::Record { val: record, .. } => (record.cols, vec![record.vals]),
+        Value::Record { val: record, .. } => {
+            let (key, val) = record.into_iter().unzip();
+            (key, vec![val])
+        }
         Value::List { vals, .. } => {
             let mut columns = get_columns(&vals);
             let data = convert_records_to_dataset(&columns, vals);

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

@@ -1041,21 +1041,9 @@ fn set_config(hm: &mut HashMap<String, Value>, path: &[&str], value: Value) -> b
     match val {
         Value::Record { val: record, .. } => {
             if path.len() == 2 {
-                if record.cols.len() != record.vals.len() {
-                    return false;
-                }
+                let key = path[1];
 
-                let key = &path[1];
-
-                let pos = record.cols.iter().position(|v| v == key);
-                match pos {
-                    Some(i) => {
-                        record.vals[i] = value;
-                    }
-                    None => {
-                        record.push(*key, value);
-                    }
-                }
+                record.insert(key, value);
             } else {
                 let mut hm2: HashMap<String, Value> = HashMap::new();
                 for (k, v) in record.iter() {

crates/nu-explore/src/views/record/mod.rs

@@ -708,10 +708,7 @@ fn build_table_as_list(v: &RecordView) -> Value {
         .cloned()
         .map(|vals| {
             Value::record(
-                Record {
-                    cols: headers.clone(),
-                    vals,
-                },
+                Record::from_raw_cols_vals(headers.clone(), vals),
                 NuSpan::unknown(),
             )
         })
@@ -726,7 +723,7 @@ fn build_table_as_record(v: &RecordView) -> Value {
     let cols = layer.columns.to_vec();
     let vals = layer.records.get(0).map_or(Vec::new(), |row| row.clone());
 
-    Value::record(Record { cols, vals }, NuSpan::unknown())
+    Value::record(Record::from_raw_cols_vals(cols, vals), NuSpan::unknown())
 }
 
 fn report_cursor_position(mode: UIMode, cursor: XYCursor) -> String {