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Merges two changesets.

This function merges two changesets provided they have been applied to the
same model (their `:model` field is equal); if the models differ, an
`ArgumentError` exception is raised. If one of the changesets has a `:repo`
field which is not `nil`, then the value of that field is used as the `:repo`
field of the resulting changeset; if both changesets have a non-`nil` and
different `:repo` field, an `ArgumentError` exception is raised.

The other fields are merged with the following criteria:

* `params` - params are merged (not deep-merged) giving precedence to the
  params of `changeset2` in case of a conflict. If both changesets have their
  `:params` fields set to `nil`, the resulting changeset will have its params
  set to `nil` too.
* `changes` - changes are merged giving precedence to the `changeset2`
* `errors` and `validations` - they are simply concatenated.
* `required` and `optional` - they are merged; all the fields that appear
  in the optional list of either changesets and also in the required list of
  the other changeset are moved to the required list of the resulting

## Examples

    iex> changeset1 = cast(%{title: "Title"}, %Post{}, ~w(title), ~w(body))
    iex> changeset2 = cast(%{title: "New title", body: "Body"}, %Post{}, ~w(title body), ~w())
    iex> changeset = merge(changeset1, changeset2)
    iex> changeset.changes
    %{body: "Body", title: "New title"}
    iex> changeset.required
    [:title, :body]
    iex> changeset.optional

    iex> changeset1 = cast(%{title: "Title"}, %Post{body: "Body"}, ~w(title), ~w(body))
    iex> changeset2 = cast(%{title: "New title"}, %Post{}, ~w(title), ~w())
    iex> merge(changeset1, changeset2)
    ** (ArgumentError) different models when merging changesets
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