Rewrite x509::wrap_in_sequence to be more general

Actually, it's only slightly more complex to make this
work for all possible usize values.  So let's do that.

rustls/src/x509.rs

@@ -7,15 +7,15 @@ fn wrap_in_asn1_len(bytes: &mut Vec<u8>) {
 
     if len <= 0x7f {
         bytes.insert(0, len as u8);
-    } else if len <= 0xff {
-        bytes.insert(0, 0x81u8);
-        bytes.insert(1, len as u8);
-    } else if len <= 0xffff {
-        bytes.insert(0, 0x82u8);
-        bytes.insert(1, ((len >> 8) & 0xff) as u8);
-        bytes.insert(2, (len & 0xff) as u8);
     } else {
-        assert!(len <= 0xffff, "excessively long x.509 name");
+        bytes.insert(0, 0x80u8);
+        let mut left = len;
+        while left > 0 {
+            let byte = (left & 0xff) as u8;
+            bytes.insert(1, byte);
+            bytes[0] += 1;
+            left >>= 8;
+        }
     }
 }
 
@@ -72,3 +72,23 @@ fn test_huge() {
                val[..6].to_vec());
     assert_eq!(val.len(), 0xffff + 4);
 }
+
+#[test]
+fn test_gigantic() {
+    let mut val = Vec::new();
+    val.resize(0x100000, 0x12);
+    wrap_in_sequence(&mut val);
+    assert_eq!(vec![0x30, 0x83, 0x10, 0x00, 0x00, 0x12, 0x12],
+               val[..7].to_vec());
+    assert_eq!(val.len(), 0x100000 + 5);
+}
+
+#[test]
+fn test_ludicrous() {
+    let mut val = Vec::new();
+    val.resize(0x1000000, 0x12);
+    wrap_in_sequence(&mut val);
+    assert_eq!(vec![0x30, 0x84, 0x01, 0x00, 0x00, 0x00, 0x12, 0x12],
+               val[..8].to_vec());
+    assert_eq!(val.len(), 0x1000000 + 6);
+}