Extend #[deriving(...)] helpers to expose field attributes

[erickt]

This pull request adds field attribute support to the syntax::ext::deriving library. This allows #[deriving(...)] handlers to work with attributes. For example, this could enable the #[deriving(Encodable, Decodable)] to customize how a field name is serialized, which is useful if implementing a schema that uses a field name that is a rust keyword.

This also includes some minor cleanup of a couple bits of libsyntax.

[brson]

cc @huonw@nikomatsakis Somebody should have an opinion on this.

[lilyball]

Can this get a docstring?

[lilyball]

I like this. We still need to figure out a generic way of customizing deriving, but having access to field attributes is going to be necessary.

[erickt]

@kballard: thanks for checking it out. I'm going to close this now, because I think I found a better way to do this that should also simplify down how to write #[deriving(...)] syntax extensions. I'm hoping to push up that PR later on tonight.