Prefer `Module#const_get(class_name)` over `eval(class_name)`

Before

  $ time bin/inch ../rails/activerecord/
  real  0m55.688s
  user  0m55.540s
  sys 0m0.170s

After

  $ time bin/inch ../rails/activerecord/
  real  0m54.399s
  user  0m54.289s
  sys 0m0.134s