Prefer `String#lines` in favor of `String.split`

This commit improves performance. Example

  yard -qn --no-save --no-cache rails

Before

  real  1m37.673s
  user  1m37.260s
  sys 0m0.284s

After

  real  1m26.865s
  user  1m26.528s
  sys 0m0.236s