Use the default recursion_limit.

Now that `extension_trait!` is gone, an increased limit isn't necessary.

src/lib.rs

@@ -282,7 +282,6 @@
 #![doc(test(attr(deny(rust_2018_idioms, warnings))))]
 #![doc(test(attr(allow(unused_extern_crates, unused_variables))))]
 #![doc(html_logo_url = "https://async.rs/images/logo--hero.svg")]
-#![recursion_limit = "2048"]
 
 extern crate alloc;