Split `asset_type` to `H160` and `ShardId`.

Currently, `asset_type` is a 256-bit hash with the prefix(`'S'` and shard id).
Actually, it is the same with the address of the asset scheme.
But we don't need to use the address as an asset type.
It is sufficient to be able to compute the address from the asset type.
So I propose to use the blake160 hash of the mint transaction tracker as an asset type and add a `shard_id` field.