Store on-chain data graph

[aaronc]

Feature: Store on-chain data graph
As an Regen Ledger user
I want to be able to store an RDF data graph
Because I want this data indexed in the global claim database

Given an RDF data graph
When I submit a valid MsgStoreData transaction
Then I should get back the URL for the on-chain data graph

• wrote docs
• wrote tests

DEFINITIONS and RATIONALE:
RDF data graph - a format for data graphs that allows indexing in a data store which can use the Sparql query language to query over it. This is advantageous because Sparql has the right features to make it usable as a language for contracts - flexible query expressions over heterogenous data, logic-programming based, and the ability to query external data stores in a single query (Sparql SERVICE)
Global claim database - Regen Ledger provides a global claim database. The data for a claim is represented as an RDF data graph (see W3C Verifiable Claims for some prior work this design is based on). So these on-chain data graphs form the based of the global claim database. The database would also include signatures/verifications of claims but that is covered in other modules

DEV NOTES:

• MsgStoreData should include the both the data in n-triples format and the hash of the data. The hash should be similar to URDNA2015, but for performance we'll use Blake2b-256 and make blank nodes illegal so we just need to verify that the triples are alphabetized.
• The data should be identified internally by a data.Address which is the hash of the data plus a one-byte prefix for the type of hash/data storage
• The URL should be the bech32 encoding of the data.Address with the prefix xrn:d/
• The goal is to index in a Sparql data store but that will be covered in another issue

[aaronc]

TODO: relate this to #18 instead of n-triples