Custom default resolvers

[bruce]

Resolves #77.

Adds a macro, default_resolve, than can be set in a schema to override the built-in default resolve function.

Eg, to support string [instead of atom] keyed maps:

default_resolve fn
  _, %{source: source, definition: %{name: name}} ->
    case source do
      %{} ->
        {:ok, Map.get(source, name)}
       _ ->
        {:ok, nil}
    end
end

@benwilson512 Would appreciate a sanity and performance pass. I've tried to limit the overhead, but it's likely you'll find some improvements with some benchmarking.

[bruce]

To support adding, eg, deprecation notices