Stabilize Option::filter.

Fixes rust-lang#45860

Author: tmccombs

src/libcore/option.rs

@@ -639,7 +639,7 @@ impl<T> Option<T> {
     /// assert_eq!(Some(4).filter(is_even), Some(4));
     /// ```
     #[inline]
-    #[unstable(feature = "option_filter", issue = "45860")]
+    #[stable(feature = "option_filter", since = "1.27.0")]
     pub fn filter<P: FnOnce(&T) -> bool>(self, predicate: P) -> Self {
         if let Some(x) = self {
             if predicate(&x) {