jid: Make the crate no_std

Only std::error::Error is missing in this mode, but that’s only waiting
for its stabilisation, see this issue:
https://github.com/rust-lang/rust/issues/103765

jid/src/error.rs

@@ -8,8 +8,9 @@
 // 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 core::fmt;
+#[cfg(feature = "std")]
 use std::error::Error as StdError;
-use std::fmt;
 
 /// An error that signifies that a `Jid` cannot be parsed from a string.
 #[derive(Debug, PartialEq, Eq)]
@@ -49,6 +50,7 @@ pub enum Error {
     ResourceInBareJid,
 }
 
+#[cfg(feature = "std")]
 impl StdError for Error {}
 
 impl fmt::Display for Error {