A union of two or more class arms and at most two unit arms (`A | B`,
`A | B | undefined`, sum types over classes) now holds the instance
pointer instead of a reference-counted %ScrUnion box, as unions of
records already do. NULL and the null sentinel encode the unit arms.
The arm of a value is the arm whose preorder interval holds the
instance's class id. On 64-bit targets, hierarchy members already keep
their class id in the high half of the rc word, or in the 32-bit header
of ProgramLifetime classes. A standalone class that is an arm of such a
union now keeps its id the same way and counts in the low half; the
count reaches the immortal all-ones value instead of wrapping, as for
hierarchy members.
Each union has a tag helper that narrowing checks inline. Unions over
the same classes share one retain, release and trace: retain and
release use the shared 32-bit count, and only a release of the last
reference dispatches on the arm to that class's release. Unions over
ProgramLifetime classes retain and release nothing. When one arm
carries a collector header, every class of every arm's hierarchy does.
Arms may not extend one another, and must all be compact or all keep
the rc word, and all be ProgramLifetime classes or none. 32-bit
targets, library builds, lifetime-region programs, debug builds, and
unions used as Map keys or Set elements keep boxes.
