input: Write a python function to register a user into the 'users' table.
  assert register_user('Alice') is None
  assert register_user('Bob') is None
  
  Write a python function to make a reservation for a product.
  assert make_reservation(1, 'Alice', '2023-10-01 10:00:00') is None
  assert make_reservation(2, 'Bob', '2023-10-02 11:00:00') is None
output: import sqlite3
  
  def register_user(username):
      conn = sqlite3.connect('database.db')
      cursor = conn.cursor()
      cursor.execute('CREATE TABLE IF NOT EXISTS users (username TEXT PRIMARY KEY)')
      cursor.execute('INSERT INTO users (username) VALUES (?)', (username,))
      conn.commit()
      conn.close()
  
  def make_reservation(product_id, username, reservation_time):
      conn = sqlite3.connect('database.db')
      cursor = conn.cursor()
      cursor.execute('CREATE TABLE IF NOT EXISTS reservations (product_id INTEGER, username TEXT, reservation_time TEXT)')
      cursor.execute('INSERT INTO reservations (product_id, username, reservation_time) VALUES (?, ?, ?)', (product_id, username, reservation_time))
      conn.commit()
      conn.close()