The key should be changed to extended_key. Otherwise, it will fail to compile.
let (extended_key, derivation) = key_chain.derive_private_key("m/1H/0".into()).expect("derive ExtendedPrivKey");
let key = BitcoinPrivKey { network: BitcoinNetwork::MainNet, derivation, extended_key, };

The secp256k1 crate updated and has some more developed features, including using a much newer version of the rand crate. It would be nice if we could switch to that to avoid duplicating dependencies.

I've been looking for a way to derive for paths like m/44'/9004'/0'/0 and I can't work out how to reach there using this lib.
Do you have an example on how to do it?

The actual version of the project is pointing to ring = "0.14.0" which has a dependency to spin = "0.5.0" and this version has a vulnerability problem.
https://github.com/jjyr/hdwallet/blob/master/Cargo.toml#L20

It's possible to update the ring to ring = "0.16.9" which is using the spin = "0.5.2"
https://github.com/briansmith/ring/blob/master/Cargo.toml#L307

error: Vulnerable crates found!

ID:	 RUSTSEC-2019-0013
Crate:	 spin
Version: 0.5.0
Date:	 2019-08-27
URL:	 https://github.com/mvdnes/spin-rs/issues/65
Title:	 Wrong memory orderings in RwLock potentially violates mutual exclusion
Solution: upgrade to: >= 0.5.2

seed in:

is a 0-lenght Vec, rng.fill(seed.as_mut_slice()); is a noop (you get a mutable 0-size slice).

Proper approach is to use a zero-ed vector (vec![0; len]) or after creating a Vec with capacity to use the unsafe set_len (although you have to make sure the bytes aren't being read, as it opens you up to potential memory exploits if they are).