Add support for defining custom map types

[solnic]

This builds on top of the custom type API and allows you to define a custom map type, like this:

defmodule User do
  use Drops.Type, %{
    required(:name) => string(),
    optional(:age) => integer()
  }
end

defmodule AccountContract do
  use Drops.Contract

  schema do
    %{
      required(:account) => %{
        required(:user) => User
      }
    }
  end
end