Implement Extend and FromIterator for OsString

Add the following trait impls:

- `impl Extend<OsString> for OsString`
- `impl<'a> Extend<&'a OsStr> for OsString`
- `impl FromIterator<OsString> for OsString`
- `impl<'a> FromIterator<&'a OsStr> for OsString`

Because `OsString` is a platform string with no particular semantics,
concatenating them together seems acceptable.

library/std/src/ffi/os_str.rs

@@ -5,6 +5,7 @@ use crate::borrow::{Borrow, Cow};
 use crate::cmp;
 use crate::fmt;
 use crate::hash::{Hash, Hasher};
+use crate::iter::{Extend, FromIterator};
 use crate::ops;
 use crate::rc::Rc;
 use crate::str::FromStr;
@@ -1182,3 +1183,47 @@ impl FromStr for OsString {
         Ok(OsString::from(s))
     }
 }
+
+#[stable(feature = "osstring_extend", since = "1.52.0")]
+impl Extend<OsString> for OsString {
+    #[inline]
+    fn extend<T: IntoIterator<Item = OsString>>(&mut self, iter: T) {
+        for s in iter {
+            self.push(&s);
+        }
+    }
+}
+
+#[stable(feature = "osstring_extend", since = "1.52.0")]
+impl<'a> Extend<&'a OsStr> for OsString {
+    #[inline]
+    fn extend<T: IntoIterator<Item = &'a OsStr>>(&mut self, iter: T) {
+        for s in iter {
+            self.push(s);
+        }
+    }
+}
+
+#[stable(feature = "osstring_extend", since = "1.52.0")]
+impl FromIterator<OsString> for OsString {
+    #[inline]
+    fn from_iter<I: IntoIterator<Item = OsString>>(iter: I) -> Self {
+        let mut buf = Self::new();
+        for s in iter {
+            buf.push(&s);
+        }
+        buf
+    }
+}
+
+#[stable(feature = "osstring_extend", since = "1.52.0")]
+impl<'a> FromIterator<&'a OsStr> for OsString {
+    #[inline]
+    fn from_iter<I: IntoIterator<Item = &'a OsStr>>(iter: I) -> Self {
+        let mut buf = Self::new();
+        for s in iter {
+            buf.push(s);
+        }
+        buf
+    }
+}