Add Macro.decompose_call/1

Hereby an implementation of Macro.decompose_call/1, as discussed on elixir-lang-core (see: https://groups.google.com/d/msg/elixir-lang-core/RaJYbKBB7vU/Ff3HTECx8pgJ )

We didn't decided on the final name, so if you think a different name should be used, I'll update the pull request.