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.
Is it really worth being pedantic about what "the heap" means, rather than just using it as a loose synonym for non-stack dynamic allocations, which is definitely the common case. (How else would one write the "boxes are allocated on the heap" sentence that conveys the same sense?)