collections: Allow inlining str::as_bytes and str::slice_unchecked

These are crucial to inline, and are alreary marked inline in libcore.
Because neither `impl str {` nor these methods are generic, the explicit
attribute is needed.

Author: Ulrik Sverdrup

src/libcollections/str.rs

@@ -1153,6 +1153,7 @@ impl str {
     ///     assert_eq!(s.slice_unchecked(0, 21), "Löwe 老虎 Léopard");
     /// }
     /// ```
+    #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub unsafe fn slice_unchecked(&self, begin: usize, end: usize) -> &str {
         core_str::StrExt::slice_unchecked(self, begin, end)
@@ -1462,6 +1463,7 @@ impl str {
     /// ```
     /// assert_eq!("bors".as_bytes(), b"bors");
     /// ```
+    #[inline]
     #[stable(feature = "rust1", since = "1.0.0")]
     pub fn as_bytes(&self) -> &[u8] {
         core_str::StrExt::as_bytes(self)