feat(grid): implement cell random generation

Prefer filled over empty cells when generating them randomly because it
makes the game more fun.

src/grid.rs

@@ -1,3 +1,5 @@
+mod rand;
+
 #[derive(Clone, Default, Debug, PartialEq)]
 pub enum Cell {
     /// Filled.

src/grid/rand.rs

@@ -0,0 +1,14 @@
+use crate::grid::Cell;
+use rand::distributions::{Distribution, Standard};
+use rand::Rng;
+
+impl Distribution<Cell> for Standard {
+    fn sample<R: Rng + ?Sized>(&self, rng: &mut R) -> Cell {
+        let filled = rng.gen_bool(2. / 3.);
+        if filled {
+            Cell::Filled
+        } else {
+            Cell::Empty
+        }
+    }
+}