Changes:
  + Implement `Focus.set/3` with `Focus.over/3` (thanks @pmeinhardt)
  6d37f88
  + Fix for attempting to compose multiple non-existing lenses to set a
  value f2a36c0
  + Fix for setting values in maps when the current value is `nil` -
  ca5d524

Notes:
  + Attempting to view or update values using lenses that aren't present
  in the structure now invariably returns `{:error, {:lens,
  :bad_path}}`. In some cases previously, the original structure would
  be returned unmodified. The new behavior is more consistent.