Implement Default for OsRng, JitterRng and ThreadRng

Also implement `ThreadRng::new`.

Refs rust-random#378.

src/jitter.rs

@@ -727,6 +727,12 @@ impl JitterRng {
     }
 }
 
+impl Default for JitterRng {
+    fn default() -> JitterRng {
+        JitterRng::new().expect("failed to initialize `JitterRng`")
+    }
+}
+
 #[cfg(feature="std")]
 mod platform {
     #[cfg(not(any(target_os = "macos", target_os = "ios", target_os = "windows",

src/os.rs

@@ -71,6 +71,12 @@ impl OsRng {
     }
 }
 
+impl Default for OsRng {
+    fn default() -> OsRng  {
+        OsRng::new().expect("failed to initialize `OsRng`")
+    }
+}
+
 impl RngCore for OsRng {
     fn next_u32(&mut self) -> u32 {
         impls::next_u32_via_fill(self)

src/thread_rng.rs

@@ -98,6 +98,21 @@ pub fn thread_rng() -> ThreadRng {
     ThreadRng { rng: THREAD_RNG_KEY.with(|t| t.clone()) }
 }
 
+impl ThreadRng {
+    /// See [`thread_rng`].
+    ///
+    /// [`thread_rng`]: fn.thread_rng.html
+    pub fn new() -> ThreadRng {
+        thread_rng()
+    }
+}
+
+impl Default for ThreadRng {
+    fn default() -> ThreadRng {
+        ThreadRng::new()
+    }
+}
+
 impl RngCore for ThreadRng {
     #[inline(always)]
     fn next_u32(&mut self) -> u32 {