Fixed enable the mutation of a unmarshaled json.

[mabels]

  In the decode methods the nested orderedMaps had
  not been pointer with prevents the mutation.
  To the fix broke some tests the cast to x.(OrderedMap)
  failed. To Fix this the OrderedMap is now an interface.
  This broke the TestUnmarshalJSONStruct test to fix
  the export of OrderedMapImpl is need. This change
  is a breaking!

Thank you for awesome work --- and sorry for breaking things

meno

[funjon]

Can confirm, this PR fixes the issues I had with nested JSON. If you can merge it, that'd be super helpful!