conn: Add noCopy

Closes #349

Author: nhooyr

Diff for: conn.go

@@ -42,6 +42,8 @@ const (
 // This applies to context expirations as well unfortunately.
 // See https://github.com/nhooyr/websocket/issues/242#issuecomment-633182220
 type Conn struct {
+	noCopy
+
 	subprotocol    string
 	rwc            io.ReadWriteCloser
 	client         bool
@@ -288,3 +290,6 @@ func (m *mu) unlock() {
 	default:
 	}
 }
+
+type noCopy struct{}
+func (*noCopy) Lock() {}

Diff for: ws_js.go

@@ -40,6 +40,7 @@ const (
 
 // Conn provides a wrapper around the browser WebSocket API.
 type Conn struct {
+	noCopy
 	ws wsjs.WebSocket
 
 	// read limit for a message in bytes.
@@ -563,3 +564,6 @@ func (m *mu) unlock() {
 	default:
 	}
 }
+
+type noCopy struct{}
+func (*noCopy) Lock() {}