Part 1 of rewriting auto_encode to use deriving

Good morning,

This first patch series adds support for `#[deriving(Decodable, Encodable)]`, but does not yet remove `#[auto_encode]` and `#[auto_decode]`. I need a snapshot to remove the old code. Along the way it also extends support for tuple structs and struct enum variants.

Also, it includes a minor fix to the pretty printer. We decided a while ago to use 4 spaces to indent a match arm instead of 2. This updates the pretty printer to reflect that.