The user-facing changes effected by this commit are:
* The 'http::Cookies<'_>' guard is now '&http::CookieJar<'_>'.
* The "one-at-a-time" jar restriction is no longer imposed.
* 'CookieJar' retrieval methods return 'http::CookieCrumb'.
* The 'private-cookies' feature is now called 'secrets'.
* Docs flag private cookie methods with feature cfg.
* Local, async request dispatching is never serialized.
* 'Client::cookies()' returns the tracked 'CookieJar'.
* 'LocalResponse::cookies()' returns a 'CookieJar'.
* 'Response::cookies()' returns an 'impl Iterator'.
* A path of '/' is set by default on all cookies.
* 'SameSite=strict' is set by default on all cookies.
* 'LocalRequest::cookies()' accepts any 'Cookie' iterator.
* The 'Debug' impl for 'Request' prints the cookie jar.
Resolves#1332.
Sergio Benitez