slice: #[inline] a couple iterator methods.

The one I care about and actually saw in the wild not getting inlined is
clone(). We ended up doing a whole function call for something that just
copies two pointers.

I ended up marking as_slice / as_ref as well because make_slice is
inline(always) itself, and is also the kind of think that can kill
performance in hot loops if you expect it to get inlined. But happy to
undo those.

library/core/src/slice/iter.rs

@@ -127,6 +127,7 @@ impl<'a, T> Iter<'a, T> {
     /// ```
     #[must_use]
     #[stable(feature = "iter_to_slice", since = "1.4.0")]
+    #[inline]
     pub fn as_slice(&self) -> &'a [T] {
         self.make_slice()
     }
@@ -146,13 +147,15 @@ iterator! {struct Iter -> *const T, &'a T, const, {/* no mut */}, {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl<T> Clone for Iter<'_, T> {
+    #[inline]
     fn clone(&self) -> Self {
         Iter { ptr: self.ptr, end: self.end, _marker: self._marker }
     }
 }
 
 #[stable(feature = "slice_iter_as_ref", since = "1.13.0")]
 impl<T> AsRef<[T]> for Iter<'_, T> {
+    #[inline]
     fn as_ref(&self) -> &[T] {
         self.as_slice()
     }
@@ -303,13 +306,15 @@ impl<'a, T> IterMut<'a, T> {
     /// ```
     #[must_use]
     #[stable(feature = "slice_iter_mut_as_slice", since = "1.53.0")]
+    #[inline]
     pub fn as_slice(&self) -> &[T] {
         self.make_slice()
     }
 }
 
 #[stable(feature = "slice_iter_mut_as_slice", since = "1.53.0")]
 impl<T> AsRef<[T]> for IterMut<'_, T> {
+    #[inline]
     fn as_ref(&self) -> &[T] {
         self.as_slice()
     }