Update to axum-core 0.1

This changes tower-cookies to have a public dependency on axum-core
instead of axum.

axum-core is designed to be a small crate that only contains the
FromRequest and IntoResponse traits. Because of the smaller API its
less likely to receive breaking changes. Therefore its recommended for
library to depend on that instead of axum.

Note this is a breaking change since axum 0.3 doesn't know about
axum-core.

Thank you @davidpdrsn 🙏

Signed cookies

async fn handler(cookies: Cookies) -> String {
    cookies.signed(&my_key).get("my_cookie").unwrap().value.into()
}

Optimization

Fix: changed flag was unnecessary enabled on Cookies::get

Examples and docs are adapted to Axum-0.3

v0.2.1

Examples and docs is adapted to Axum-0.3

No need for `Cookies` to be `mut` anymore

• Cookies methods now take &self instead of &mut self.
  Migration: fn handler(mut cookies: Cookies) => fn handler(cookies: Cookies)
• Standard layer semantic.
  Migration: .layer(CookieLayer) => .layer(CookieManagerLayer::new())

Thank you @programatik29 🙏