Remove pieces of code which index slices by shard id

[jancionear]

Many places in the code index slices by shard id, like this:

let prev_chunk_proof = prev_chunk_proofs[shard_id as usize].clone();

This is a risky thing to do, as an invalid shard id could cause a panic.
It would be safer to convert all such pieces of code so that they don't panic when they encounter an invalid shard_id:

- let prev_chunk_proof = prev_chunk_proofs[shard_id as usize].clone();
+ let prev_chunk_proof = prev_chunk_proofs
+                    .get(shard_id as usize)
+                    .ok_or(Error::InvalidShardId(shard_id))?
+                    .clone();

Stateless validation and resharding will introduce a lot of new code which will use shard ids in new ways, often dealing with untrusted data, and it's easy to accidentally use a function that doesn't handle invalid shard ids. It would be safer to just eradicate all such functions.
It would give a peace of mind that there won't be any issues like #10621