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.

src/unit.rs

@@ -5,14 +5,14 @@ use log::{debug, info};
 use qstring::QString;
 use url::Url;
 
-#[cfg(feature = "cookie")]
+#[cfg(feature = "cookies")]
 use cookie::Cookie;
 
 use crate::body::{self, BodySize, Payload, SizedReader};
 use crate::header;
 use crate::resolve::ArcResolver;
 use crate::stream::{self, connect_test, Stream};
-#[cfg(feature = "cookie")]
+#[cfg(feature = "cookies")]
 use crate::Agent;
 use crate::{Error, Header, Request, Response};
 
@@ -81,7 +81,7 @@ impl Unit {
                 extra.push(Header::new("Authorization", &format!("Basic {}", encoded)));
             }
 
-            #[cfg(feature = "cookie")]
+            #[cfg(feature = "cookies")]
             extra.extend(extract_cookies(&req.agent, &url).into_iter());
 
             extra
@@ -94,7 +94,7 @@ impl Unit {
             .cloned()
             .collect();
 
-        let deadline = match req.timeout {
+        let deadline = match req.agent.config.timeout {
             None => None,
             Some(timeout) => {
                 let now = time::Instant::now();
@@ -203,12 +203,12 @@ pub(crate) fn connect(
     };
 
     // squirrel away cookies
-    #[cfg(feature = "cookie")]
+    #[cfg(feature = "cookies")]
     save_cookies(&unit, &resp);
 
     // handle redirects
-    if resp.redirect() && req.redirects > 0 {
-        if redirect_count == req.redirects {
+    if resp.redirect() && req.agent.config.redirects > 0 {
+        if redirect_count == req.agent.config.redirects {
             return Err(Error::TooManyRedirects);
         }
 
@@ -255,11 +255,11 @@ pub(crate) fn connect(
     Ok(resp)
 }
 
-#[cfg(feature = "cookie")]
+#[cfg(feature = "cookies")]
 fn extract_cookies(agent: &Agent, url: &Url) -> Option<Header> {
     let header_value = agent
         .state
-        .jar
+        .cookie_tin
         .get_request_cookies(url)
         .iter()
         .map(|c| c.encoded().to_string())
@@ -295,7 +295,7 @@ fn connect_socket(unit: &Unit, hostname: &str, use_pooled: bool) -> Result<(Stre
         while let Some(stream) = agent
             .state
             .pool
-            .try_get_connection(&unit.url, &unit.req.proxy)
+            .try_get_connection(&unit.url, unit.req.agent.config.proxy.clone())
         {
             let server_closed = stream.server_closed()?;
             if !server_closed {
@@ -379,7 +379,7 @@ fn send_prelude(unit: &Unit, stream: &mut Stream, redir: bool) -> io::Result<()>
 }
 
 /// Investigate a response for "Set-Cookie" headers.
-#[cfg(feature = "cookie")]
+#[cfg(feature = "cookies")]
 fn save_cookies(unit: &Unit, resp: &Response) {
     //
 
@@ -397,7 +397,7 @@ fn save_cookies(unit: &Unit, resp: &Response) {
     unit.req
         .agent
         .state
-        .jar
+        .cookie_tin
         .store_response_cookies(cookies, &unit.url.clone());
 }
 
@@ -411,13 +411,13 @@ mod tests {
 
     #[test]
     fn match_cookies_returns_one_header() {
-        let agent = Agent::default();
+        let agent = Agent::new();
         let url: Url = "https://crates.io/".parse().unwrap();
         let cookie1: Cookie = "cookie1=value1; Domain=crates.io; Path=/".parse().unwrap();
         let cookie2: Cookie = "cookie2=value2; Domain=crates.io; Path=/".parse().unwrap();
         agent
             .state
-            .jar
+            .cookie_tin
             .store_response_cookies(vec![cookie1, cookie2].into_iter(), &url);
 
         // There's no guarantee to the order in which cookies are defined.