std: Stabilize into_* ASCII methods

These were intended to land in stable 1.8 but were just waiting for the
implementation PR, so now they're landing. Specifically this PR stabilizes:

* `AsciiExt::into_ascii_uppercase`
* `AsciiExt::into_ascii_lowercase`
* `AsciiExt for Vec<u8>`
* `AsciiExt for String`

src/libstd/ascii.rs

@@ -179,7 +179,7 @@ pub trait AsciiExt {
     ///
     /// assert_eq!(upper, "A");
     /// ```
-    #[unstable(feature = "ascii", issue = "27809")]
+    #[stable(feature = "into_ascii", since = "1.8.0")]
     fn into_ascii_uppercase(self) -> Self::Owned where Self: Sized {
         self.to_ascii_uppercase()
     }
@@ -202,15 +202,15 @@ pub trait AsciiExt {
     ///
     /// assert_eq!(lower, "a");
     /// ```
-    #[unstable(feature = "ascii", issue = "27809")]
+    #[stable(feature = "into_ascii", since = "1.8.0")]
     fn into_ascii_lowercase(self) -> Self::Owned where Self: Sized {
         self.to_ascii_lowercase()
     }
 }
 
 /// Implement `into_ascii_lowercase` and `into_ascii_uppercase` without memory allocation,
 /// defer other methods to `str`.
-#[unstable(feature = "ascii", issue = "27809")]
+#[stable(feature = "into_ascii", since = "1.8.0")]
 impl AsciiExt for String {
     type Owned = Self;
 
@@ -242,7 +242,7 @@ impl AsciiExt for String {
 
 /// Implement `into_ascii_lowercase` and `into_ascii_uppercase` without memory allocation,
 /// defer other methods to `[u8]`.
-#[unstable(feature = "ascii", issue = "27809")]
+#[stable(feature = "into_ascii", since = "1.8.0")]
 impl AsciiExt for Vec<u8> {
     type Owned = Self;