I used `type.cast(value)` to emulate unchecked serialized value in
`unboundable?`, since `RangeError` was raised only for the integer type,
so the emulation works enough for the integer type.

But since #41516, Enum types are also not always serializable, so
`type.cast(value)` may also be called for the enum types.

I've delegated `type.cast(value)` to the subtype if an unknown label is
passed to work the emulation even on Enum types in 3b6461bb. But it is
strange to delegate to the subtype for the emulation only if an unknown
label is passed.

Instead of using `type.cast(value)` for the emulation, extend
`serializable?` to get unchecked serialized value if the value is not
serializable.