Rollup merge of #25158 - koute:master, r=alexcrichton

I was profiling my code again and this time AsRef<str> for String
was eating up a considerable chunk of my runtime; adding the inline
annotation made the program run almost twice as fast!

While I was at it I also added the annotation to other implementations
of AsRef as well as AsMut.

src/libcollections/string.rs

@@ -1052,6 +1052,7 @@ impl<T: fmt::Display + ?Sized> ToString for T {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl AsRef<str> for String {
+    #[inline]
     fn as_ref(&self) -> &str {
         self
     }

src/libcore/convert.rs

@@ -173,6 +173,7 @@ impl<T> AsMut<[T]> for [T] {
 
 #[stable(feature = "rust1", since = "1.0.0")]
 impl AsRef<str> for str {
+    #[inline]
     fn as_ref(&self) -> &str {
         self
     }