make Parser::push more flexible

src/roxy.rs

@@ -23,8 +23,8 @@ impl<'a> Parser<'a> {
         Parser { steps: Vec::new() }
     }
 
-    pub fn push<P: Parse>(&mut self, parser: &'a mut P) {
-        self.steps.push(parser);
+    pub fn push<P: Into<&'a mut dyn Parse>>(&mut self, parser: P) {
+        self.steps.push(parser.into());
     }
 }