Move all *Rng wrappers to rngs dir

Author: pitdicker

src/distributions/float.rs

@@ -148,7 +148,7 @@ float_impls! { f64, u64, 52, 1023 }
 mod tests {
     use Rng;
     use distributions::{Open01, OpenClosed01};
-    use mock::StepRng;
+    use rngs::mock::StepRng;
 
     const EPSILON32: f32 = ::core::f32::EPSILON;
     const EPSILON64: f64 = ::core::f64::EPSILON;

src/distributions/mod.rs

@@ -479,7 +479,7 @@ fn ziggurat<R: Rng + ?Sized, P, Z>(
 #[cfg(test)]
 mod tests {
     use Rng;
-    use mock::StepRng;
+    use rngs::mock::StepRng;
     use super::{WeightedChoice, Weighted, Distribution};
 
     #[test]