* Make `frequency` detect negative frequencies, frequency
  total overflow, and zero total frequency.

* Rewrite `frequency` to build an `IntMap` to represent the
  frequency list, which greatly improves efficiency for long
  lists.

* Bump `containers` lower bound to 0.5.11.

Fixes hedgehogqa#337.

* Make the `IntMap` one smaller and hold the last element
  as a fall-through.

* Use a lazy `IntMap` instead of a strict one; there's no
  sense in forcing such polymorphic things.