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

This commit improves performance. Example

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

Before

  real  0m28.925s
  user  0m28.760s
  sys 0m0.124s

After

  real  0m25.489s
  user  0m25.296s
  sys 0m0.132s