syntax: deprecate #[auto_{en,de}code] in favour of #[deriving({En,De}codable)].

Replace all instances of #[auto_*code] with the appropriate #[deriving] attribute
and remove the majority of the actual auto_* code, leaving stubs to refer the user to
the new syntax.

Also, moves the useful contents of auto_encode.rs to more appropriate spots: tests and comments to deriving/encodable.rs, and the ExtCtxtMethods trait to build.rs (unused so far, but the method syntax might be nicer than using the mk_* fns in many instances).