Create Record type (#10103)

# Description
This PR creates a new `Record` type to reduce duplicate code and
possibly bugs as well. (This is an edited version of #9648.)
- `Record` implements `FromIterator` and `IntoIterator` and so can be
iterated over or collected into. For example, this helps with
conversions to and from (hash)maps. (Also, no more
`cols.iter().zip(vals)`!)
- `Record` has a `push(col, val)` function to help insure that the
number of columns is equal to the number of values. I caught a few
potential bugs thanks to this (e.g. in the `ls` command).
- Finally, this PR also adds a `record!` macro that helps simplify
record creation. It is used like so:
   ```rust
   record! {
       "key1" => some_value,
       "key2" => Value::string("text", span),
       "key3" => Value::int(optional_int.unwrap_or(0), span),
       "key4" => Value::bool(config.setting, span),
   }
   ```
Since macros hinder formatting, etc., the right hand side values should
be relatively short and sweet like the examples above.

Where possible, prefer `record!` or `.collect()` on an iterator instead
of multiple `Record::push`s, since the first two automatically set the
record capacity and do less work overall.

# User-Facing Changes
Besides the changes in `nu-protocol` the only other breaking changes are
to `nu-table::{ExpandedTable::build_map, JustTable::kv_table}`.

crates/nu-command/src/generators/cal.rs

@@ -359,18 +359,7 @@ fn add_month_to_table(
             day_number += 1;
         }
 
-        let cols: Vec<String> = indexmap.keys().map(|f| f.to_string()).collect();
-        let mut vals: Vec<Value> = Vec::new();
-        for c in &cols {
-            if let Some(x) = indexmap.get(c) {
-                vals.push(x.to_owned())
-            }
-        }
-        calendar_vec_deque.push_back(Value::Record {
-            cols,
-            vals,
-            span: tag,
-        })
+        calendar_vec_deque.push_back(Value::record(indexmap.into_iter().collect(), tag))
     }
 
     Ok(())