Change advance(_back)_by to return usize instead of Result<(), usize>

A successful advance is now signalled by returning `0` and other values now represent the remaining number
of steps that couldn't be advanced as opposed to the amount of steps that have been advanced during a partial advance_by.

This simplifies adapters a bit, replacing some `match`/`if` with arithmetic. Whether this is beneficial overall depends
on whether `advance_by` is mostly used as a building-block for other iterator methods and adapters or whether
we also see uses by users where `Result` might be more useful.

library/core/src/array/iter.rs

@@ -284,12 +284,11 @@ impl<T, const N: usize> Iterator for IntoIter<T, N> {
         self.next_back()
     }
 
-    fn advance_by(&mut self, n: usize) -> Result<(), usize> {
-        let original_len = self.len();
-
+    fn advance_by(&mut self, n: usize) -> usize {
         // This also moves the start, which marks them as conceptually "dropped",
         // so if anything goes bad then our drop impl won't double-free them.
         let range_to_drop = self.alive.take_prefix(n);
+        let remaining = n - range_to_drop.len();
 
         // SAFETY: These elements are currently initialized, so it's fine to drop them.
         unsafe {
@@ -297,7 +296,7 @@ impl<T, const N: usize> Iterator for IntoIter<T, N> {
             ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(slice));
         }
 
-        if n > original_len { Err(original_len) } else { Ok(()) }
+        remaining
     }
 }
 
@@ -334,12 +333,11 @@ impl<T, const N: usize> DoubleEndedIterator for IntoIter<T, N> {
         })
     }
 
-    fn advance_back_by(&mut self, n: usize) -> Result<(), usize> {
-        let original_len = self.len();
-
+    fn advance_back_by(&mut self, n: usize) -> usize {
         // This also moves the end, which marks them as conceptually "dropped",
         // so if anything goes bad then our drop impl won't double-free them.
         let range_to_drop = self.alive.take_suffix(n);
+        let remaining = n - range_to_drop.len();
 
         // SAFETY: These elements are currently initialized, so it's fine to drop them.
         unsafe {
@@ -347,7 +345,7 @@ impl<T, const N: usize> DoubleEndedIterator for IntoIter<T, N> {
             ptr::drop_in_place(MaybeUninit::slice_assume_init_mut(slice));
         }
 
-        if n > original_len { Err(original_len) } else { Ok(()) }
+        remaining
     }
 }