This adds a `getSpareBits` method to all the TypeInfo classes
that returns a suitable bitmask indicating the spare bits available
in values of this type. This gives us a way to recursively explore
the type tree and build up a full spare bit mask for an arbitrary type.
Happily, it appears we actually do have enough information to do this
calculation entirely from first principles, without requiring additional
reflection information. So once this is stable, we should remove my
earlier incomplete effort to publish spare bit mask info in the reflection
data, as that adds unnecessary metadata to every binary.
This doubtless still has plenty of holes, but seems sufficient
to handle a few basic enum types, including the stdlib DecodingError
which was used as an example to work out some key issues.
