Add a skeleton Data Representation chapter

[asajeffrey]

Fixes #22

[asajeffrey]

Some comments...

• File an issue for an introduction?
• Should structs and tuples be merged?
• Do we want to say anything about other representations? (e.g. Box, slices, fat pointers, ...)
• Do we want an ABI section?

[avadacatavra]

I'd say we can keep tuples and structs together at the moment. If it seems like they need to diverge, that can be taken care of when @nikomatsakis does the writeups