allow on-the-fly migration of primitive vs. boxed fields

[c247t]

Establishes wire compatibility of boxed and primitive types of the same content, obsoleting the need for declaring a new field and bridging with wrappers. The @externalize annotation has been enhanced to provide fine-grained control, defaulting to established practice.

The pLongNullValue attributes et.al. define the primitive value to be equivalent to null, which is implicitly applied to the field whenever null is read from the input.
The sparseBoxed attribute defines whether this conversion is to be mirrored when converting from primitive to boxed values, i.e. null is applied to the field whenever the corresponding value is encountered.

All null-equivalent values default to zero / false, while sparse boxing is enabled by default.