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/math/min.rs

@@ -2,7 +2,9 @@ use crate::math::reducers::{reducer_for, Reduce};
 use crate::math::utils::run_with_function;
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Category, Example, PipelineData, ShellError, Signature, Span, Type, Value};
+use nu_protocol::{
+    Category, Example, PipelineData, Record, ShellError, Signature, Span, Type, Value,
+};
 
 #[derive(Clone)]
 pub struct SubCommand;
@@ -50,11 +52,10 @@ impl Command for SubCommand {
             Example {
                 description: "Compute the minima of the columns of a table",
                 example: "[{a: 1 b: 3} {a: 2 b: -1}] | math min",
-                result: Some(Value::Record {
+                result: Some(Value::test_record(Record {
                     cols: vec!["a".to_string(), "b".to_string()],
                     vals: vec![Value::test_int(1), Value::test_int(-1)],
-                    span: Span::test_data(),
-                }),
+                })),
             },
             Example {
                 description: "Find the minimum of a list of arbitrary values (Warning: Weird)",