Redo field traversal

[phemmer]

Right now, deStruct traverses fields recursively, makes copies, scalars, and children objects. This is rather difficult, and complex. We really shouldn't try to maintain near identical copies of the input data. Instead we should convert it to a basic, but simple interface all handlers can consume.

One option:
Redo this so that there are no copies, and all objects are converted into a field type. The field should have:

• Type method which indicates the original type of the field
• Name the key for the field
• Scalar method which returns a scalar value
• String method which returns a string of the scalar value
• SubFields method which returns a nested map of sub fields
• Fields method which returns a flattened map of sub fields, including the field itself
• MarshalJSON self explanatory