Configurable storage hasher

[xlc]

I am updating the storage hasher for our ORML open-web3-stack/open-runtime-module-library#101

For the oracle module, I am using twox64_concat for T::OracleKey, which is an enum for Acala and LamianrChain, so it is safe as there will be only < 10 possible values.

However this may cause issue if someone is using u128 as T::OracleKey.

Without the ability to configure hasher, it is not possible to implement efficient and reusable pallet.

[gui1117]

to make hasher configurable in the current macro we could change decl_storage to get a type instead of one of the defined variant as such:

decl_storage! {
	trait Store for Module<T: Trait> as System {
		Account: map hasher(MyHasher) T::AccountId => ();
	}
}

MyHasher will then have to implement StorageHasher
and also we have to add a metadata function (probably in StorageHasher trait) to retrieve metadata at runtime.