Expose list of atoms

[wolflee]

Exposing a list of atoms as in https://github.com/elixir-maru/maru_entity#example does not work.

For example,

defmodule PostEntity do
  use Maru.Entity

  expose [:id, :title]
  expose :comments, using: List[CommentEntity], batch: PostCommentBatchHelper
end

will cause compilation error like:

== Compilation error in file lib/entity/serializers.ex ==
** (FunctionClauseError) no function clause matching in Maru.Entity.expose/1
    expanding macro: Maru.Entity.expose/1
    lib/entity/serializers.ex:19: PostEntity (module)
    (elixir) lib/kernel/parallel_compiler.ex:198: anonymous fn/4 in Kernel.ParallelCompiler.spawn_workers/6