message: Make it more clear record has fixed number of fields
  Summary: For the following diff
diff: @@ -289,23 +289,24 @@ where
  let fields = record_fields(RecordType::from_value(this).unwrap());
  let mut values = Vec::with_capacity(fields.len());
  for (name, field) in fields.iter() {
  - match field.0.default {
  + let value = match field.0.default {
  None => {
  let v: Value = param_parser.next(name)?;
  v.check_type_compiled(field.0.typ, &field.1, Some(name))?;
  - values.push(v);
  + v
  }
  Some(default) => {
  let v: Option<Value> = param_parser.next_opt(name)?;
  match v {
  - None => values.push(default),
  + None => default,
  Some(v) => {
  v.check_type_compiled(field.0.typ, &field.1, Some(name))?;
  - values.push(v);
  - }
  + v
  }
  }
  }
  + };
  + values.push(value);
  }
  Ok(eval.heap().alloc_complex(Record { typ: this, values }))
  })
  