Ownership guide

This is a work in progress, but this should get *extensive* review, so I'm putting it up early and often.

This is the start of a draft of the new 'ownership guide,' which explains ownership, borrowing, etc. I'm feeling better about this framing than last time's, but we'll see.
As someone else said, it's not just count that matters, they have to be specifically paired with each allocation.

> We always have a exactly one deallocation paired with each of our allocations.