Better branch likelyness on stable

It uses uses the `#[cold]` attribute to get a similar effect

Author: a1phyr

src/raw/mod.rs

@@ -43,14 +43,23 @@ use self::imp::Group;
 // consistently improves performance by 10-15%.
 #[cfg(feature = "nightly")]
 use core::intrinsics::{likely, unlikely};
+
+#[cfg(not(feature = "nightly"))]
+#[inline]
+#[cold]
+fn cold() {}
+
 #[cfg(not(feature = "nightly"))]
 #[inline]
 fn likely(b: bool) -> bool {
+    if !b { cold() }
     b
 }
 #[cfg(not(feature = "nightly"))]
+#[cold]
 #[inline]
 fn unlikely(b: bool) -> bool {
+    if b { cold() }
     b
 }