fixup! Fix #330: Display quick stats 📊

Fix these Clippy warnings:

cargo clippy
    Checking mcfly v0.8.6 (/Users/nicolas/work/mcfly)
warning: useless use of `format!`
  --> src/stats_generator.rs:26:24
   |
26 |         lines.push_str(format!("📊 Quick stats:\n").as_mut_str());
   |                        ^^^^^^^^^^^^^^^^^^^^^^^^^^^^ help: consider using `.to_string()`: `"📊 Quick stats:\n".to_string()`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#useless_format
   = note: `#[warn(clippy::useless_format)]` on by default

warning: deref which would be done by auto-deref
  --> src/stats_generator.rs:29:24
   |
29 |           lines.push_str(&*Self::generate_command_stats(
   |  ________________________^
30 | |             self,
31 | |             limit,
32 | |             most_used_commands,
33 | |         ));
   | |_________^
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#explicit_auto_deref
   = note: `#[warn(clippy::explicit_auto_deref)]` on by default
help: try
   |
29 ~         lines.push_str(&Self::generate_command_stats(
30 +             self,
31 +             limit,
32 +             most_used_commands,
33 ~         ));
   |

warning: accessing first element with `vec.get(0)`
  --> src/stats_generator.rs:82:9
   |
82 |         vec.get(0).unwrap().count
   |         ^^^^^^^^^^ help: try: `vec.first()`
   |
   = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#get_first
   = note: `#[warn(clippy::get_first)]` on by default

warning: `mcfly` (lib) generated 3 warnings (run `cargo clippy --fix --lib

src/stats_generator.rs

@@ -23,10 +23,10 @@ impl<'a> StatsGenerator<'a> {
         if count_history == 0 {
             return "No history, no stats!".to_string();
         }
-        lines.push_str(format!("📊 Quick stats:\n").as_mut_str());
+        lines.push_str("📊 Quick stats:\n");
         lines.push_str(format!("  - history has {:?} items ;\n", count_history).as_mut_str());
         let most_used_commands = self.most_used_commands(&limit);
-        lines.push_str(&*Self::generate_command_stats(
+        lines.push_str(&Self::generate_command_stats(
             self,
             limit,
             most_used_commands,
@@ -79,7 +79,7 @@ impl<'a> StatsGenerator<'a> {
             .run_query("select count(1) as n from commands", &[], |row| {
                 Ok(Count { count: row.get(0)? })
             });
-        vec.get(0).unwrap().count
+        vec.first().unwrap().count
     }
 }