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.
Boxes manage a dynamic allocation from a memory allocator. The language does not specify where it comes from. On Linux, jemalloc uses anonymous memory mappings rather than the data section the OS refers to as the heap. I already [filed a bug](https://github.com/rust-lang/rust/issues/18226) about this issue in the documentation.