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-engine/src/column.rs

@@ -19,8 +19,11 @@ pub fn get_columns(input: &[Value]) -> Vec<String> {
 }
 
 // If a column doesn't exist in the input, return it.
-pub fn nonexistent_column(inputs: &[String], columns: &[String]) -> Option<String> {
-    let set: HashSet<String> = HashSet::from_iter(columns.iter().cloned());
+pub fn nonexistent_column<'a, I>(inputs: &[String], columns: I) -> Option<String>
+where
+    I: IntoIterator<Item = &'a String>,
+{
+    let set: HashSet<&String> = HashSet::from_iter(columns);
 
     for input in inputs {
         if set.contains(input) {