careful implementation of deepcopy

[CarloLucibello]

Should overlad deepcopy_internals:

help?> deepcopy
search: deepcopy

  deepcopy(x)

  Create a deep copy of x: everything is copied recursively, resulting in a fully independent object. For example,
  deep-copying an array produces a new array whose elements are deep copies of the original elements. Calling
  deepcopy on an object should generally have the same effect as serializing and then deserializing it.

  While it isn't normally necessary, user-defined types can override the default deepcopy behavior by defining a
  specialized version of the function deepcopy_internal(x::T, dict::IdDict) (which shouldn't otherwise be used),
  where T is the type to be specialized for, and dict keeps track of objects copied so far within the recursion.
  Within the definition, deepcopy_internal should be used in place of deepcopy, and the dict variable should be
  updated as appropriate before returning.