Extend #[deriving(Default)] to support specified field defaults

When deriving the Default trait, individual struct fields can be marked
up with a `#[default="expr"]` attribute to indicate that this field
should be given the results of `expr` instead of `Default::default()`.

Example:

    #[deriving(Default)]
    struct Foo {
        #[default="42"]
        x: int,
        #[default="Ok(())"]
        err: IoResult<()>
    }