Implement inclusion proof of a contract in contracts tree

[benesjan]

The code should do the following:

1. Implement oracle which returns preimage of a contracts tree leaf,
2. check the relevant preimage components (e.g. bytecode, construct args etc.)
3. hash the preimage and check that it matches the contracts tree leaf,
4. prove that the leaf is in the contracts tree at a given block,
5. do the rest the same way as for notes etc.