Move Juno examples into juno_samples

juno_frontend/examples/matmul.jn

-fn matmul<n : usize, m : usize, l : usize>(a : f32[n, m], b : f32[m, l]) -> f32[n, l] {
-  let res : f32[n, l];
-
-  @outer for i = 0 to n {
-    @middle for j = 0 to l {
-      @inner for k = 0 to m {
-        res[i, j] += a[i, k] * b[k, j];
-      }
-    }
-  }
-
-  @exit return res;
-}