feat(lib): remove extern Url type usage

BREAKING CHANGE: The `Url` type is no longer used. Any instance in the
  `Client` API has had it replaced with `hyper::Uri`.

  This also means `Error::Uri` has changed types to
  `hyper::error::UriError`.

  The type `hyper::header::parsing::HTTP_VALUE` has been made private,
  as an implementation detail. The function `http_percent_encoding`
  should be used instead.

examples/client.rs

@@ -24,8 +24,8 @@ fn main() {
         }
     };
 
-    let url = hyper::Url::parse(&url).unwrap();
-    if url.scheme() != "http" {
+    let url = url.parse::<hyper::Uri>().unwrap();
+    if url.scheme() != Some("http") {
         println!("This example only works with 'http' URLs.");
         return;
     }