Impl FusedIterator for record iterators (#12542)

This is good practice as all our iterators will never return a value
after reaching `None`

The benefit should be minimal as only `Iterator::fuse` is directly
specialized and itself rarely used (sometimes in `itertools` adaptors)

Thus it is mostly a documentation thing

crates/nu-protocol/src/value/record.rs

@@ -1,4 +1,4 @@
-use std::ops::RangeBounds;
+use std::{iter::FusedIterator, ops::RangeBounds};
 
 use crate::{ShellError, Span, Value};
 
@@ -409,6 +409,8 @@ impl ExactSizeIterator for IntoIter {
     }
 }
 
+impl FusedIterator for IntoIter {}
+
 impl IntoIterator for Record {
     type Item = (String, Value);
 
@@ -449,6 +451,8 @@ impl<'a> ExactSizeIterator for Iter<'a> {
     }
 }
 
+impl FusedIterator for Iter<'_> {}
+
 impl<'a> IntoIterator for &'a Record {
     type Item = (&'a String, &'a Value);
 
@@ -489,6 +493,8 @@ impl<'a> ExactSizeIterator for IterMut<'a> {
     }
 }
 
+impl FusedIterator for IterMut<'_> {}
+
 impl<'a> IntoIterator for &'a mut Record {
     type Item = (&'a String, &'a mut Value);
 
@@ -529,6 +535,8 @@ impl<'a> ExactSizeIterator for Columns<'a> {
     }
 }
 
+impl FusedIterator for Columns<'_> {}
+
 pub struct IntoColumns {
     iter: std::vec::IntoIter<(String, Value)>,
 }
@@ -557,6 +565,8 @@ impl ExactSizeIterator for IntoColumns {
     }
 }
 
+impl FusedIterator for IntoColumns {}
+
 pub struct Values<'a> {
     iter: std::slice::Iter<'a, (String, Value)>,
 }
@@ -585,6 +595,8 @@ impl<'a> ExactSizeIterator for Values<'a> {
     }
 }
 
+impl FusedIterator for Values<'_> {}
+
 pub struct IntoValues {
     iter: std::vec::IntoIter<(String, Value)>,
 }
@@ -613,6 +625,8 @@ impl ExactSizeIterator for IntoValues {
     }
 }
 
+impl FusedIterator for IntoValues {}
+
 pub struct Drain<'a> {
     iter: std::vec::Drain<'a, (String, Value)>,
 }
@@ -641,6 +655,8 @@ impl ExactSizeIterator for Drain<'_> {
     }
 }
 
+impl FusedIterator for Drain<'_> {}
+
 #[macro_export]
 macro_rules! record {
     // The macro only compiles if the number of columns equals the number of values,