This allows for validating multiple fields with the same validator function with the same options for all fields.

Example

iex> types = %{first_name: :string, last_name: :string}
iex> params = %{first_name: "John", last_name: "Doe"}
iex> Ecto.Changeset.cast({%{}, types}, params, Map.keys(types))
...> |> validate_many(
...>   [:first_name, :last_name],
...>   &Ecto.Changeset.validate_length/3,
...>   min: 2,
...>   max: 20
...> )
#Ecto.Changeset<action: nil, changes: %{first_name: "John", last_name: "Doe"}, errors: [], data: %{}, valid?: true>

iex> types = %{first_name: :string, last_name: :string}
iex> params = %{first_name: "J", last_name: "DoppelgängerDoppelgänger"}
iex> Ecto.Changeset.cast({%{}, types}, params, Map.keys(types))
...> |> validate_many(
...>   [:first_name, :last_name],
...>   &Ecto.Changeset.validate_length/3,
...>   min: 2,
...>   max: 20,
...>   message: "this field is invalid"
...> )
#Ecto.Changeset<action: nil, changes: %{first_name: "J", last_name: "DoppelgängerDoppelgänger"}, errors: [last_name: {"this field is invalid", [count: 20, validation: :length, kind: :max, type: :string]}, first_name: {"this field is invalid", [count: 2, validation: :length, kind: :min, type: :string]}], data: %{}, valid?: false>