feat(http1): Make HTTP/1 support an optional feature

cc #2251

BREAKING CHANGE: This puts all HTTP/1 methods and support behind an
  `http1` cargo feature, which will not be enabled by default. To use
  HTTP/1, add `features = ["http1"]` to the hyper dependency in your
  `Cargo.toml`.

src/cfg.rs

@@ -1,9 +1,37 @@
-macro_rules! cfg_http2 {
+macro_rules! cfg_any_http {
     ($($item:item)*) => {
         $(
-            #[cfg(feature = "http2")]
-            //#[cfg_attr(docsrs, doc(cfg(feature = "http2")))]
+            #[cfg(any(
+                feature = "http1",
+                feature = "http2",
+            ))]
+            #[cfg_attr(docsrs, doc(cfg(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")))]
+                $item
+            )*
+        }
+    }
+
+    macro_rules! cfg_http2 {
+        ($($item:item)*) => {
+            $(
+                #[cfg(feature = "http2")]
+                #[cfg_attr(docsrs, doc(cfg(feature = "http2")))]
+                $item
+            )*
+        }
+    }
+}