Finish low-level record / link patterns

[pospi]

This branch feels like it's stabilising and ready for feedback, there are some cleanup jobs I want to log but will do that separately. This PR:

• Changes zome names to be type-based. Moves observation.main to observation.economic_event.
• Adds basic intent capability to planning DNA. Moves planning.main to planning.commitment and adds planning.intent.
• Wraps up common record management behaviour into a set of helpers in lib/hdk_graph_helpers/src/record_helpers.rs
• Proposes a pattern for implementing & using traits that manage record behaviour, see lib/hdk_graph_helpers/src/record_interface.rs
• Wraps up common link management behaviour into a set of helpers in lib/hdk_graph_helpers/src/link_helpers.rs

This will close #18, #19, #20 & #23. #21 can probably be closed and ignored in lieu of finishing all fields for the main flow record types as its logic is no longer any different to #20. Will log the larger job separately when I transcribe my cleanup notes.

AFAIK more or less the only remaining cleanup to get to a nice pattern for the rest of the system is the record macros in #22, which as you can see when you look at intent.rs, commitment.rs & economic_event.rs is simply a case of wrapping the commonalities between those 3 files up.

Does not yet, but needs to:

• Be able to deserialize the results of cross-zome & cross-DNA method calls into structs in order to intelligently handle them (see MM thread)