Switched over from OsRng --> thread_rng

- At least on my Linux machine, a signiciant amount of time (> 50%) was spent inside
  OsRng.
- Likely due to blocking behaviour of the rng devices on Linux.
- thread_rng should not block, but at the same time should provide good
  enough rng.

src/field/field.rs

@@ -267,7 +267,7 @@ pub trait Field:
     }
 
     fn rand() -> Self {
-        Self::rand_from_rng(&mut OsRng)
+        Self::rand_from_rng(&mut rand::thread_rng())
     }
 
     fn rand_vec(n: usize) -> Vec<Self> {