parsers: Replace std stuff with alloc/core stuff

parsers/src/date.rs

@@ -4,8 +4,8 @@
 // License, v. 2.0. If a copy of the MPL was not distributed with this
 // file, You can obtain one at http://mozilla.org/MPL/2.0/.
 
-use std::borrow::Cow;
-use std::str::FromStr;
+use alloc::borrow::Cow;
+use core::str::FromStr;
 
 use xso::{error::Error, AsXmlText, FromXmlText};