Implement slice::{Iter, IterMut}::{advance_by, advance_back_by}

Co-authored-by: The8472 <git@infinite-source.de>

Author: timvermeulen

library/core/src/slice/iter/macros.rs

@@ -185,6 +185,14 @@ macro_rules! iterator {
                 }
             }
 
+            #[inline]
+            fn advance_by(&mut self, n: usize) -> Result<(), usize>{
+                let advance = cmp::min(n, len!(self));
+                // SAFETY: `advance` does not exceed `self.len()` by construction
+                unsafe { self.post_inc_start(advance as isize) };
+                if advance == n { Ok(()) } else { Err(advance) }
+            }
+
             #[inline]
             fn last(mut self) -> Option<$elem> {
                 self.next_back()
@@ -371,6 +379,14 @@ macro_rules! iterator {
                     Some(next_back_unchecked!(self))
                 }
             }
+
+            #[inline]
+            fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
+                let advance = cmp::min(n, len!(self));
+                // SAFETY: `advance` does not exceed `self.len()` by construction
+                unsafe { self.pre_dec_end(advance as isize) };
+                if advance == n { Ok(()) } else { Err(advance) }
+            }
         }
 
         #[stable(feature = "fused", since = "1.26.0")]