chancell: PaddingNegotiate: Provide two constructors, not one

We don't really want the caller to pass ignored timeout parameters.
And this makes more semantic sense.

The stop constructor uses zero, which is what C Tor does.  See
  https://gitlab.torproject.org/tpo/core/torspec/-/merge_requests/76

crates/tor-cell/src/chancell/msg.rs

@@ -816,18 +816,25 @@ pub struct PaddingNegotiate {
     ito_high_ms: u16,
 }
 impl PaddingNegotiate {
-    /// Create a new PaddingNegotiate message.
-    ///
-    /// If `start` is true, this is a message to enable padding. Otherwise
-    /// this is a message to disable padding.
-    pub fn new(start: bool, ito_low_ms: u16, ito_high_ms: u16) -> Self {
-        let command = if start { 2 } else { 1 };
+    /// Create a new PADDING_NEGOTIATE START message.
+    pub fn start(ito_low_ms: u16, ito_high_ms: u16) -> Self {
+        // Tor Spec section 7.3
         Self {
-            command,
+            command: 2,
             ito_low_ms,
             ito_high_ms,
         }
     }
+
+    /// Create a new PADDING_NEGOTIATE STOP message.
+    pub fn stop() -> Self {
+        // Tor Spec section 7.3
+        Self {
+            command: 1,
+            ito_low_ms: 0,
+            ito_high_ms: 0,
+        }
+    }
 }
 impl Body for PaddingNegotiate {
     fn into_message(self) -> ChanMsg {

crates/tor-cell/tests/testvec_chanmsg.rs

@@ -395,7 +395,13 @@ fn test_padding_negotiate() {
     fbody(
         cmd,
         "00 02 0100 0200",
-        &msg::PaddingNegotiate::new(true, 256, 512).into(),
+        &msg::PaddingNegotiate::start(256, 512).into(),
+    );
+
+    fbody(
+        cmd,
+        "00 01 0000 0000",
+        &msg::PaddingNegotiate::stop().into(),
     );
 
     assert_eq!(