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.
Short version: maybe use something like "By convention, lifetimes in cases like this (where you can see the lifetime and what it's attached to in the signature) are given one-letter names, ...".

This sets off a warning klaxon in my head: it's encouraging people to use non-descriptive, single-letter names everywhere.  If I had a dollar for every time I had to lambaste a student for that... well, I'd be much richer. :P

I don't know what the "official" position on this is, but my rule of thumb has been leaning toward only using single letters when the purpose of the lifetime is completely obvious from context (usually functions that don't qualify for elision).  In places like traits or structs (not relevant right now, admittedly), I try to give them meaningful names.  There's already been a few times I've read through the library docs and not had any idea what a particular lifetime argument is for.