Add a internal type: VkArguments

[gcao]

This can be useful when we want to allow constructing same arguments and using in multiple places.

...
case kind: ValueKind
of VkArguments
  arg_props: Table[string, Value]
  arg_children: seq[Value]

Or

Arguments = ref object
  props: Table[string, Value]
  children: seq[Value]
...
case kind: ValueKind
of VkArguments
  arguments: Arguments