Fix no_std for idna (#843)

Co-authored-by: Luca Casonato <hello@lcas.dev>

idna/src/uts46.rs

@@ -453,7 +453,7 @@ impl Idna {
             return Errors::default();
         }
         let mut errors = processing(domain, self.config, &mut self.normalized, out);
-        self.output = std::mem::replace(out, String::with_capacity(out.len()));
+        self.output = core::mem::replace(out, String::with_capacity(out.len()));
         let mut first = true;
         for label in self.output.split('.') {
             if !first {