better cookie support

[siddhantk232]

• feat/cookies #19 (comment)
• feat/cookies #19 (comment)

Also, not happy with the RC<RefCell<Vec<Cookie>>> type for In::set_cookies. Should replace this with a blanket CookieStore and move the In::add_cookie() method there.
Functions that want to manipulate cookies should then take CookieStore and not the whole In this way we avoid unnecessary copy.

[amitu]

IntoCookie

impl In {
    // note that this is private
    fn add_cookie_raw(&self, s: String) {
         self.cookies.push(s)
    }

    pub fn add_cookie<K: AsRef<str>, V: AsRef<str>(&self, key: K, value: V) {}
}

pub trait IntoCookie {
    fn to_cookie(&self) -> String;
    fn add_cookie_into(in_: &In) {
        in_.add_cookie_raw(self.to_cookie())
    }
}

// someday if we decide to expose our own cookie struct
impl IntoCookie for OurCookie {}

// we will support cookie crate behind a feature flag
#[cfg(feature = "cookie")]
impl IntoCookie for cookie::Cookie {}