trait-based serialization and auto-serialization

[erickt]

This adds trait-based serialization and auto-serialization. It's parallel with the old serialization library to help with the migration. This has the same approach as the old function-based one, just with traits. Because of this, we can simplify down auto-serialization to handling just records, structs, and enums. Everything else can be implemented in the library.

There are some downsides to the trait-based approach though:

• We lose support for auto-serializing arbitrary-sized tuples. Instead we must extend serialization2.rs if we want to automatically support more than 5 argument tuples.
• Pretty much every use of #[auto_serialize] requires this preamble at the top of the file, which would be nice to auto-generate.

extern mod std;
use std::serialization2::{Serializable, deserializer}

• I have no idea how to support mutable fields. I have a cut down version of my problem here. I can't throw pure everywhere, because serializers might not be pure. I could change the serialize signature to fn serialize<S: Serializer>(&mut self) { ... } but then everything that will be serialized must be stored in a let mut ... slot. Oddly enough the old function-based approach does not have this problem.

[erickt]

I just noticed that generic types aren't properly auto-serializing. Regular types seem fine. I'll try to get that fixed and add some tests tomorrow.

[erickt]

I've fixed the generic type issues, added temporary prettyprint2 and ebml2 libraries, and a test. We'll need a snapshot before I can convert everything over to the trait-based approach.

[graydon]

Rebased and landed, thanks.