The storage runtime interface should not enforce a hash type (#4231)

* The storage runtime interface should not enforce a hash type

Currently the runtime interface enforces `H256` as hash type, but in the
future people could use whatever they want as hash type. The hash type
always needs to match between the runtime and the node, but that is
already required.

* Update primitives/externalities/src/lib.rs

Co-Authored-By: thiolliere <gui.thiolliere@gmail.com>

substrate/primitives/state-machine/src/changes_trie/build.rs

@@ -18,7 +18,7 @@
 
 use std::collections::{BTreeMap, BTreeSet};
 use std::collections::btree_map::Entry;
-use codec::Decode;
+use codec::{Decode, Encode};
 use hash_db::Hasher;
 use num_traits::One;
 use crate::backend::Backend;
@@ -47,6 +47,7 @@ pub(crate) fn prepare_input<'a, B, H, Number>(
 	where
 		B: Backend<H>,
 		H: Hasher + 'a,
+		H::Out: Encode,
 		Number: BlockNumber,
 {
 	let number = parent.number.clone() + One::one();
@@ -200,7 +201,7 @@ fn prepare_digest_input<'a, H, Number>(
 	), String>
 	where
 		H: Hasher,
-		H::Out: 'a,
+		H::Out: 'a + Encode,
 		Number: BlockNumber,
 {
 	let build_skewed_digest = config.end.as_ref() == Some(&block);