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.

README.md

@@ -61,9 +61,7 @@ You can control them when including `ureq` as a dependency.
 ```
 
 * `tls` enables https. This is enabled by default.
-* `native-tls` enables https using the [`native-tls`](https://crates.io/crates/native-tls) crate. 
-  NB: To make this work you currently need to use `default-features: false` to disable `tls`.
-  We plan on fixing that.
+* `cookies` enables handling cookies between requests in an agent.
 * `json` enables `response.into_json()` and `request.send_json()` via serde_json.
 * `charset` enables interpreting the charset part of
   `Content-Type: text/plain; charset=iso-8859-1`. Without this, the library