improve memory initialization

src/cpu.rs

@@ -94,9 +94,9 @@ struct Instruction {
 
 impl Ram {
     fn new() -> Self {
-        let mut mem = Vec::new();
-        mem.resize((RAM_SIZE - RAM_BASE) as usize, 0);
-        Self { mem }
+        Self {
+            mem: vec![0; (RAM_SIZE - RAM_BASE) as usize],
+        }
     }
 }