feat(client): Make client an optional feature

cc #2223

BREAKING CHANGE: The HTTP client of hyper is now an optional feature. To
  enable the client, add `features = ["client"]` to the dependency in
  your `Cargo.toml`.

src/cfg.rs

@@ -1,37 +1,50 @@
-macro_rules! cfg_any_http {
-    ($($item:item)*) => {
+macro_rules! cfg_feature {
+    (
+        #![$meta:meta]
+        $($item:item)*
+    ) => {
         $(
-            #[cfg(any(
-                feature = "http1",
-                feature = "http2",
-            ))]
-            #[cfg_attr(docsrs, doc(cfg(any(
-                feature = "http1",
-                feature = "http2",
-            ))))]
+            #[cfg($meta)]
+            #[cfg_attr(docsrs, doc(cfg($meta)))]
             $item
         )*
     }
 }
 
+macro_rules! cfg_any_http {
+    ($($item:item)*) => {
+        cfg_feature! {
+            #![any(feature = "http1", feature = "http2")]
+            $($item)*
+        }
+    }
+}
+
 cfg_any_http! {
     macro_rules! cfg_http1 {
         ($($item:item)*) => {
-            $(
-                #[cfg(feature = "http1")]
-                #[cfg_attr(docsrs, doc(cfg(feature = "http1")))]
-                $item
-            )*
+            cfg_feature! {
+                #![feature = "http1"]
+                $($item)*
+            }
         }
     }
 
     macro_rules! cfg_http2 {
         ($($item:item)*) => {
-            $(
-                #[cfg(feature = "http2")]
-                #[cfg_attr(docsrs, doc(cfg(feature = "http2")))]
-                $item
-            )*
+            cfg_feature! {
+                #![feature = "http2"]
+                $($item)*
+            }
+        }
+    }
+
+    macro_rules! cfg_client {
+        ($($item:item)*) => {
+            cfg_feature! {
+                #![feature = "client"]
+                $($item)*
+            }
         }
     }
 }