Expand Host Contracts Visibility

The host module now has a method Contracts() which is supported by the API, and by siac. It's all experimental, and we're not committing to compatibility for this. We want to get the first commit into the master branch though, I guess I'll open that in a different PR.

With this command you can see all of the contracts that the host has formed and has failed to form, as well as some information like how much potential revenue there is, and how many sectors there are.