Move as many data structures from `Value` to `SchemeCompatible` as possible

[maplant]

We only want to have 8 values so that we can have a reasonable size for fix numbers without allocation (61 bits). This also means that we want to promote FixNumbers to a value type.

Off the top of my head, the following can be easily moved to record types:

• undefined can be moved to pair in the same way as null was
• RecordTypeDescriptor has no reason to be in Value
• Port should be a record type
• HashTable should be a record type
• Syntax might be fine to move, but need to check the performance implications
• ByteVector is probably fine to move, Vector is a bit of a stretch