Rework invariant post

[moea]

I made some far-reaching changes, as I was finding it difficult to include a lot of background while keeping the post size manageable. I settled on having the post talk almost entirely about how we're using / planning to use invariants, and targeting it at people w/ a degree of Datalog familiarity. It doesn't cover too many broader Datopia details that are covered in other posts.

We could have follow-up posts which talk about using invariant as an independent library when we've got a more solid API or other working examples - the idea with this one is to provoke interest in people w/ the relevant background knowledge.

Minor stuff:

• I shortened the balance invariant, and fixed an issue w/ bigdec/long comparison - scratch gist here
• datacoin -> lambdacoin, as there's apparently a cryptocurrency called datacoin

Let me know if you think this doesn't flow, or there's stuff you think ought to be covered.

[whilo]

Except for the typo below this is very well written and focused to target audience. Thanks for taking the time to rewrite it!

[whilo]

Datalog is deterministic