Rollup merge of rust-lang#60256 - ethanboxx:master, r=alexcrichton

Option::flatten

This PR makes this possible.
```rust
assert_eq!(Some(6), Some(Some(6)).flatten());
assert_eq!(Some(6), Some(Some(6)).into());
```

Author: Centril

src/libcore/option.rs

@@ -136,7 +136,7 @@
 #![stable(feature = "rust1", since = "1.0.0")]
 
 use crate::iter::{FromIterator, FusedIterator, TrustedLen};
-use crate::{hint, mem, ops::{self, Deref}};
+use crate::{convert, hint, mem, ops::{self, Deref}};
 use crate::pin::Pin;
 
 // Note that this is not a lang item per se, but it has a hidden dependency on
@@ -1413,3 +1413,33 @@ impl<T> ops::Try for Option<T> {
         None
     }
 }
+
+impl<T> Option<Option<T>> {
+    /// Converts from `Option<Option<T>>` to `Option<T>`
+    ///
+    /// # Examples
+    /// Basic usage:
+    /// ```
+    /// #![feature(option_flattening)]
+    /// let x: Option<Option<u32>> = Some(Some(6));
+    /// assert_eq!(Some(6), x.flatten());
+    ///
+    /// let x: Option<Option<u32>> = Some(None);
+    /// assert_eq!(None, x.flatten());
+    ///
+    /// let x: Option<Option<u32>> = None;
+    /// assert_eq!(None, x.flatten());
+    /// ```
+    /// Flattening once only removes one level of nesting:
+    /// ```
+    /// #![feature(option_flattening)]
+    /// let x: Option<Option<Option<u32>>> = Some(Some(Some(6)));
+    /// assert_eq!(Some(Some(6)), x.flatten());
+    /// assert_eq!(Some(6), x.flatten().flatten());
+    /// ```
+    #[inline]
+    #[unstable(feature = "option_flattening", issue = "60258")]
+    pub fn flatten(self) -> Option<T> {
+        self.and_then(convert::identity)
+    }
+}