PYTH12 — Python mapping issue: value boxes

I suggest a change to the mapping for value boxes, that makes them
much more convenient to use. I believe that none of the Python ORBs
implement the currently specified mapping, so changing the mapping at
this point will not affect any ORB implementations or application
code.

I suggest that the new mapping is:

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Value boxes are mapped to the normal mapping of the boxed type, or
None for null value boxes. For example, given IDL

valuetype BoxedString string;
interface I

;

the operation could be called as:

obj.example("Hello", None)

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Discussion: this mapping has one limitation, in that in a boxed object
reference type it is impossible to distinguish a null value box from a
non-null value box containing a nil object reference. This is
considered acceptable since the IDL to Java mapping has the same
property; there is a clear precedent for this kind of mapping.

Accept the proposed changes