verify local file hash, add retry, add cache busting on retry

src/misc.rs

@@ -158,3 +158,14 @@ pub fn save_cache(dir: &Path, cache: structs::Cache) {
         println_error!("Failed to save cache: {}", e);
     });
 }
+
+pub fn random_string(length: u32) -> String {
+    use rand::Rng;
+    let mut rng = rand::thread_rng();
+    let mut result = String::new();
+    for _ in 0..length {
+        let random: u8 = rng.gen_range(33..127);
+        result.push(random as char);
+    }
+    result
+}