Variable length ReceiptClaim encoding for use in Solidity

[nategraf]

One of the friction points with the Eth Verifirer API is that the user has to drag around the post_state_digest. It is a field only needed to construct the receipt claim digest, and not a detail users need to concern themselves with, yet it pollutes the API.

By using a variable length encoding, it would be possible to have a ReceiptClaim type that optionally includes various components, including the post state digest. Action here is to implement this variable length encoding, and then use it to simplify the verifier API.