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-command/src/network/http/client.rs

@@ -602,9 +602,12 @@ fn headers_to_nu(headers: &Headers, span: Span) -> Result<PipelineData, ShellErr
     for (name, values) in headers {
         let is_duplicate = vals.iter().any(|val| {
             if let Value::Record { val, .. } = val {
-                if let Some(Value::String {
-                    val: header_name, ..
-                }) = val.vals.get(0)
+                if let Some((
+                    _col,
+                    Value::String {
+                        val: header_name, ..
+                    },
+                )) = val.get_index(0)
                 {
                     return name == header_name;
                 }