style: use linked hash map

Cargo.toml

@@ -18,3 +18,4 @@ actix-web = "2"
 actix-rt = "1"
 bytes = "0.5"
 futures-core = "0.3"
+linked-hash-map = "0.5"

src/main.rs

@@ -5,9 +5,9 @@ mod error;
 
 use std::pin::Pin;
 use std::task::{ Context, Poll };
-use std::collections::HashMap;
 use bytes::Bytes;
 use clap::{ App, Arg };
+use linked_hash_map::LinkedHashMap;
 use actix_web::{ web, HttpServer, HttpRequest, HttpResponse };
 
 use error::*;
@@ -70,8 +70,8 @@ async fn main() -> std::io::Result<()> {
     .await
 }
 
-fn parse_query_string(query_string: &str) -> HashMap<String, Vec<String>> {
-    let map = HashMap::new();
-
+fn parse_query_string(query_string: &str) -> LinkedHashMap<String, Vec<String>> {
+    let map = LinkedHashMap::new();
+    // TODO ...
     map
 }