input: Write a python function to register and login users with password hashing.
  assert register('user1', 'password123') == 'Registration successful!'
  assert register('user1', 'password456') == 'Username already exists!'
  assert login('user1', 'password123') == 'Login successful!'
  assert login('user1', 'wrongpassword') == 'Wrong password!'
  assert login('user2', 'password123') == 'Username doesn\'t exist!'
output: import hashlib
  
  users = {}
  
  def hash_password(password):
      return hashlib.sha256(password.encode()).hexdigest()
  
  def register(username, password):
      if username in users:
          return 'Username already exists!'
      hashed_password = hash_password(password)
      users[username] = hashed_password
      return 'Registration successful!'
  
  def login(username, password):
      if username not in users:
          return 'Username doesn\'t exist!'
      hashed_password = hash_password(password)
      if users[username] == hashed_password:
          return 'Login successful!'
      else:
          return 'Wrong password!'