Allow child-trie-friendly proofs

[tomaka]

cc #166

At the moment, when decoding a proof, it is assumed that this proof only contains one trie, and the user is required to pass the expected trie root hash.

However, in the context of child tries, a proof can contain entries from multiple different tries (the main trie and child tries).

Normally, these tries are connected in the sense that the value of a main trie node starting with :child_storage:default: is equal to the root of another trie. However, I prefer to not put this assumption in the proof-related code.
Instead, I've adjusted the code to allow multiple different tries within the same proof, without checking whether these tries are connected to each other.

In terms of API changes, the API user no longer passes the expected trie root hash when decoding the proof, but instead passes the trie root hash whenever the proof is accessed. If the proof doesn't contain the requested trie, then None is returned, indicating an incomplete proof.