Allow structs to implement access

[drtheuns-enreach]

To keep compatibility with the old implementation, the dsl_struct has been changed to do the string->atom key conversion in all of the access functions. The only difference now should be that an unknown struct key being assigned to the struct will result in a string key being added to the map. For example:

defmodule Obj do
  use DslStruct, key: nil
end

# pseudo; assume access protocol
obj = %Obj{}
obj["key"] = 5
obj.key == 5

obj["unknown"] = 6 # AutoMap would have ignored this key
obj["unknown"] == 6