with_value and into_parts for PushBackN

[kindlychung]

Would be nice to have these implemented for PushBackN:

    /// Create a `PutBackN` along with the `value` to put back.
    #[inline]
    pub fn with_value(value: Vec<I::Item>, it: I) -> Self {
        PutBackN {
            top: value,
            iter: it,
        }
    }

    /// Split the `PutBackN` into its parts.
    #[inline]
    pub fn into_parts(self) -> (Vec<I::Item>, I) {
        let PutBackN{top, iter} = self;
        (top, iter)
    }