Make error messages from deriving(Foo) point to the deriving declaration as well as the field

Previously something like

    struct NotEq;

    #[deriving(Eq)]
    struct Error {
        foo: NotEq
    }

would just point to the `foo` field, with no mention of the
`deriving(Eq)`. With this patch, the compiler creates a note saying "in
expansion of #[deriving(Eq)]" pointing to the Eq.

(includes some cleanup/preparation.)