[ISSUE 17] implement drop_pii for arrays and maps

[oxg558]

Fixes #17 by implementing drop_pii for arrays and maps that have record values.

Also adds a topological sort to the generate_code function. This makes it easier to generate more complex schemas at runtime in tests like those written here for drop_pii.

[hpatoio]

Left a comment.

Few more generic questions:

• Can we tag a major version ? Unless there is a good reason not to do it. Asked a similar question here
• Is there anything ppl must do when updating to this new version ?

[oxg558]

There's an issue here for maps of maps: the value_clauses can also contain a helper function for an inner type which would mean the final unquote would leave the function clauses out of order, e.g.:

  def drop_pii_raw_payloads(value) when is_map(value) do
    Enum.reduce(value, %{}, fn {k, v}, acc ->
      Elixir.Map.put(acc, k, drop_pii_raw_payloads_values(v))
    end)
  end

  def drop_pii_raw_payloads_values(value) when is_map(value) do
    Enum.reduce(value, %{}, fn {k, v}, acc ->
      Elixir.Map.put(acc, k, drop_pii_raw_payloads_values_values(v))
    end)
  end

  def drop_pii_raw_payloads_values_values(%RawPayload{} = value) do
    RawPayload.drop_pii(value)
  end

  def drop_pii_raw_payloads_values_values(value) do
    value
  end

  def drop_pii_raw_payloads_values(value) do       # <----- out of order
    value
  end

This generates a compile warning