Using the bitfield generator to treat guards and badges as a union type can be convenient,
but it requires reserving a bit in the data for the bitfield run time type information.
This type information is not needed by the kernel as it knows implicitly whether the passed
data is a badge or a guard based on the kind of cap being operated on. However, with the
type information present we cannot pass a word sized piece of data to the kernel.
The solution here is to go back to using a plain seL4_Word as the type for invocations
that want a capdata and let the user either construct a badge as a plain word, or use
the seL4_CNode_CapData bitfield for constructing a guard, although they have to manually extract
the word representation out of it.
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