This job isn't quite finished because it caused me to discover bugs
related to reading `ByVal(Undef)` when a `ByValPair` is expected, e.g.
for a fat pointer. This wasn't a problem with the `None` of
`Option<Value>`, but I realized an equivalent bug existed even then,
since you could transmute a `u64` like `ByVal(Bytes(42))` to a fat
pointer type on 32-bit targets.
Likewise, you could transmute a fat pointer to `u64` and get panics
related to expecting `ByVal` but finding `ByValPair`, so the problem
goes both ways.
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