Multiple fields support

[YOSHIOKA Takuma]

Support outer type with multiple fields.

For example:

trait Validate<T: ?Sized> {
    type Error;
    fn validate(val: &T) -> Result<(), Self::Error>;
}
struct ValidatedString<V: Validate<str>> {
    #[opaque_typedef(inner)]
    inner: String,
    _v: ::std::marker::PhantomData<V>,
}

This feature may be useful with #8.

[YOSHIOKA Takuma]

This feature maybe nealy useless without #8 (generics support), but generics support will be really hard...