Refactor redundancy of TrieIdGenerate

[pepyakin]

At the moment the TrieId is required to return a :child_storage: prefix from the trie id it generated. This is not ideal because a trie_id is stored along with every instance of an account, and that means that we have extra 15 bytes of overhead.

We initially came with this approach because we needed something done quick and we were afraid of adding this prefix manually on every access to the child storage because it is more error-prone.

The ideal solution would be to perform access to the child storage via some common abstraction that will handle all this subtleties.