Fix new clippy warnings and upgrade dependencies (#169)

The new clippy release that comes with Rust 1.62.0 has some new lints
enabled by defaults. Those need to be fixed to make CI happy. Use this
opportunity for minor dependency upgrades.

src/lib.rs

@@ -117,7 +117,7 @@ pub fn next(lua: &Lua, (env, callback): (i32, Function)) -> LuaResult<bool> {
         };
 
         if let Some(mut next) = next {
-            let _call = server.stats().track_call();
+            server.stats().track_call();
 
             let method = next.method().to_string();
             let params = next
@@ -274,6 +274,8 @@ pub fn dcs_grpc(lua: &Lua) -> LuaResult<LuaTable> {
 }
 
 fn pretty_print_value(val: Value, indent: usize) -> LuaResult<String> {
+    use std::fmt::Write;
+
     Ok(match val {
         Value::Nil => "nil".to_string(),
         Value::Boolean(v) => v.to_string(),
@@ -285,14 +287,15 @@ fn pretty_print_value(val: Value, indent: usize) -> LuaResult<String> {
             let mut s = "{\n".to_string();
             for pair in t.pairs::<Value, Value>() {
                 let (key, value) = pair?;
-                s += &format!(
-                    "{}{} = {},\n",
+                let _ = writeln!(
+                    s,
+                    "{}{} = {},",
                     "  ".repeat(indent + 1),
                     pretty_print_value(key, indent + 1)?,
                     pretty_print_value(value, indent + 1)?
                 );
             }
-            s += &format!("{}}}", "  ".repeat(indent));
+            let _ = write!(s, "{}}}", "  ".repeat(indent));
             s
         }
         Value::Function(_) => "[function]".to_string(),