Implement StructCell as a NativeCell subclass
Implement StructCell as a NativeCell subclass

NativeCell is a class of Cells for holding native data. StructCells will
continue to live as a templated NativeCell intended to hold one native
struct. NativeCells can be used directly for more complex cases.

Signed-off-by: Robert Young <42c59959a3f4413d6a1b4d7b4f99db457d199e49@gmail.com>

ab-vm/ab,ab-vm/ab,ab-vm/ab,ab-vm/ab,ab-vm/ab