API changes for 2.0

* Remove Request::build
* All mutations on Request follow builder pattern

The previous `build()` on request was necessary because mutating
functions did not follow a proper builder pattern (taking `&mut self`
instead of `mut self`). With a proper builder pattern, the need for
`.build()` goes away.

* All Request body and call methods consume self

Anything which "executes" the request will now consume the `Request`
to produce a `Result<Response>`.

* Move all config from request to agent builder

Timeouts, redirect config, proxy settings and TLS config are now on
`AgentBuilder`.

* Rename max_pool_connections -> max_idle_connections
* Rename max_pool_connections_per_host ->  max_idle_connections_per_host

Consistent internal and external naming.

* Introduce new AgentConfig for static config created by builder.

`Agent` can be seen as having two parts. Static config and a mutable
shared state between all states. The static config goes into
`AgentConfig` and the mutable shared state into `AgentState`.

* Replace all use of `Default` for `new`.

Deriving or implementing `Default` makes for a secondary instantiation
API.  It is useful in some cases, but gets very confusing when there
is both `new` _and_ a `Default`. It's especially devious for derived
values where a reasonable default is not `0`, `false` or `None`.

* Remove feature native_tls, we want only native rustls.

This feature made for very clunky handling throughout the code. From a
security point of view, it's better to stick with one single TLS API.
Rustls recently got an official audit (very positive).

https://github.com/ctz/rustls/tree/master/audit

Rustls deliberately omits support for older, insecure TLS such as TLS
1.1 or RC4. This might be a problem for a user of ureq, but on balance
not considered important enough to keep native_tls.

* Remove auth and support for basic auth.

The API just wasn't enough. A future reintroduction should at least
also provide a `Bearer` mechanism and possibly more.

* Rename jar -> cookie_store
* Rename jar -> cookie_tin

Just make some field names sync up with the type.

* Drop "cookies" as default feature

The need for handling cookies is probably rare, let's not enable it by
default.

* Change all feature checks for "cookie" to "cookies"

The outward facing feature is "cookies" and I think it's better form
that the code uses the official feature name instead of the optional
library "cookies".

* Keep `set` on Agent level as well as AgentBuilder.

The idea is that an auth exchange might result in a header that need
to be set _after_ the agent has been built.

tests/https-agent.rs

@@ -11,12 +11,12 @@ fn agent_set_cookie() {
         headers: HashMap<String, String>,
     }
 
-    let agent = ureq::Agent::default().build();
+    let agent = ureq::Agent::new();
     let cookie = ureq::Cookie::build("name", "value")
         .domain("httpbin.org")
         .secure(true)
         .finish();
-    agent.set_cookie(cookie);
+    agent.set_cookie(cookie, &"https://httpbin.org/".parse().unwrap());
     let resp = agent
         .get("https://httpbin.org/get")
         .set("Connection", "close")
@@ -102,8 +102,6 @@ m0Wqhhi8/24Sy934t5Txgkfoltg8ahkx934WjP6WWRnSAu+cf+vW
 #[cfg(feature = "tls")]
 #[test]
 fn tls_client_certificate() {
-    let agent = ureq::Agent::default();
-
     let mut tls_config = rustls::ClientConfig::new();
 
     let certs = rustls::internal::pemfile::certs(&mut BADSSL_CLIENT_CERT_PEM.as_bytes()).unwrap();
@@ -116,11 +114,11 @@ fn tls_client_certificate() {
         .root_store
         .add_server_trust_anchors(&webpki_roots::TLS_SERVER_ROOTS);
 
-    let resp = agent
-        .get("https://client.badssl.com/")
+    let agent = ureq::builder()
         .set_tls_config(std::sync::Arc::new(tls_config))
-        .call()
-        .unwrap();
+        .build();
+
+    let resp = agent.get("https://client.badssl.com/").call().unwrap();
 
     assert_eq!(resp.status(), 200);
 }