Make proxy available without the feature

When we introduced the `rustls` feature, we did it like this:

```toml
[features]
proxy = ["hyper-proxy"]
rustls = ["hyper-proxy/rustls", "tokio-rustls", "hyper-rustls"]
tls = ["hyper-proxy/tls", "native-tls", "tokio-tls", "hyper-tls"]
```

A problem is hidden here: enabling an optional dependency's feature
enabled the dependency. Because of this, `hyper-proxy` is already being
downloaded and compiled, even if `proxy` isn't enabled:

```
$ cargo c
             …
    Checking hyper-proxy v0.6.0
    Checking tbot v0.5.1 (…)
```

But to be able to use it, you have to enable the `proxy` feature, which seems
odd. Since cargo doesn't provide a way to enable an optional dependency's
feature without enabling the dependency (at least yet; rust-lang/cargo#3494),
I think that it's better to always provide proxy functionality.

Cargo.toml

@@ -24,7 +24,7 @@ categories = [
 
 [dependencies]
 hyper = { version = "0.13.3", default-features = false }
-hyper-proxy = { version = "0.6", optional = true, default-features = false }
+hyper-proxy = { version = "0.6", default-features = false }
 tokio = { version = "0.2", features = ["time", "rt-core", "tcp"] }
 serde_json = "1"
 serde = { version = "1.0.34", features = ["derive"] }
@@ -41,7 +41,7 @@ tracing-futures = "0.2"
 meval = "0.2"
 
 [features]
-proxy = ["hyper-proxy"]
+proxy = []
 rustls = ["hyper-proxy/rustls", "tokio-rustls", "hyper-rustls"]
 tls = ["hyper-proxy/tls", "native-tls", "tokio-tls", "hyper-tls"]
 default = ["tls"]
@@ -80,7 +80,7 @@ required-features = ["tokio/macros"]
 
 [[example]]
 name = "proxy"
-required-features = ["tokio/macros", "proxy"]
+required-features = ["tokio/macros"]
 
 [[example]]
 name = "sticker_packs"

src/connectors.rs

@@ -11,15 +11,12 @@ use hyper_rustls::HttpsConnector;
 #[cfg(feature = "tls")]
 use hyper_tls::HttpsConnector;
 
-#[cfg(feature = "proxy")]
 pub use hyper_proxy as proxy;
-#[cfg(feature = "proxy")]
 use proxy::ProxyConnector;
 
 /// The default HTTPS connector.
 pub type Https = HttpsConnector<HttpConnector>;
 
-#[cfg(feature = "proxy")]
 /// The default proxy connector.
 pub type Proxy = ProxyConnector<Https>;
 
@@ -29,7 +26,6 @@ pub fn https() -> Https {
     HttpsConnector::new()
 }
 
-#[cfg(feature = "proxy")]
 /// Constructs a proxy connector.
 pub fn proxy(proxy: proxy::Proxy) -> Proxy {
     ProxyConnector::from_proxy(https(), proxy).unwrap_or_else(|error| {