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_plugin_example/src/example.rs

@@ -1,5 +1,5 @@
 use nu_plugin::{EvaluatedCall, LabeledError};
-use nu_protocol::Value;
+use nu_protocol::{Record, Value};
 pub struct Example;
 
 impl Example {
@@ -67,11 +67,13 @@ impl Example {
                     .map(|v| Value::int(v * i, call.head))
                     .collect::<Vec<Value>>();
 
-                Value::Record {
-                    cols: cols.clone(),
-                    vals,
-                    span: call.head,
-                }
+                Value::record(
+                    Record {
+                        cols: cols.clone(),
+                        vals,
+                    },
+                    call.head,
+                )
             })
             .collect::<Vec<Value>>();