Add #[inline] to the Into for From impl

Author: saethlin

library/core/src/convert/mod.rs

@@ -722,6 +722,7 @@ where
     ///
     /// That is, this conversion is whatever the implementation of
     /// <code>[From]&lt;T&gt; for U</code> chooses to do.
+    #[inline]
     fn into(self) -> U {
         U::from(self)
     }