Expand unknown AST calls inside schema block before walking the AST

[dvic]

This PR adds support for modules that call a macro that eventually produces an AST that we known (e.g., an ecto field call). This is an example of such a module:

  defmodule MySchema do
     use TypedEctoSchema

     import MyMacros

     typed_schema "foo" do
       MyMacros.add_field(:foo, :integer)
     end
   end

[dvic]

Moved to #18