h1: require constructor

Prevents users from writing `HttpClient {}`.

Author: Fishrock123

src/h1.rs

@@ -7,7 +7,9 @@ use http_types::StatusCode;
 
 /// Async-h1 based HTTP Client.
 #[derive(Debug)]
-pub struct H1Client {}
+pub struct H1Client {
+    _priv: (),
+}
 
 impl Default for H1Client {
     fn default() -> Self {
@@ -18,7 +20,7 @@ impl Default for H1Client {
 impl H1Client {
     /// Create a new instance.
     pub fn new() -> Self {
-        Self {}
+        Self { _priv: () }
     }
 }