input: Write a python function to concatenate a list of strings into a single string.
  assert concat_strings(["Hello", "World"]) == "HelloWorld"
  assert concat_strings(["Python", "is", "awesome"]) == "Pythonisawesome"
  assert concat_strings(["This", "is", "a", "test"]) == "Thisisatest"
output: def concat_strings(strings):
      return ''.join(strings)