Fulfillment & satisfaction APIs

[pospi]

This is mainly focused on completing #42 and passing end-to-end integration tests for both Fulfillment and Satisfaction-related functionality. It includes everything needed for a working first-pass at all flow records (these plus EconomicEvent, Commitment and Intent) and has full support in GraphQL for the related "read by ID" and relationship traversal queries.

It also adds the following updates:

• Adds get_links_and_load_entry_data helper, for reading a set of linked entries and base addresses attached to some context entry.
• Adds read_from_zome helper to wrap up error handling & response decoding for inter-DNA RPC calls
• More integration tests- list in Required integration tests and edge-cases list #2 is up to date
• Bugfixes to MaybeUndefined to allow for serialisation & request passing
• Better error messages from API deletion handlers
• Cleanup of zome API handler parameters and optimisation of memory usage
• Formalised contributor agreements a bit more (see Sign off on contributor guidelines #58)

To do:

There are a few additional cleanup tasks added to #39 - nothing major, just cleanup. My goal is to get the base system passing integration tests ASAP so that others can start developing on it before we make a serious optimisation pass. We also need to think through what we have so far and sound out any worthwhile improvements before continuing on to other modules besides observation, planning & specification.

This work also opens up some other avenues for exploration around creating shared abstractions for storing link metadata. It is quite likely that adopting an XDI format as a base for records like Fulfillment and Satisfaction would be beneficial for interoperability with projects like the Link Index DNA.

It may also be worth investigating the Holochain signalling API more as a followup- see #57.

[pospi]

I know you're all on the road right now so I'm going to self-merge this so that @philipbeadle & other devs can get started on an up-to-date system this/next week (: