tvix/eval: use ErrorKind::TvixBug more consistently

I think there are two kinds of situation where we should use TvixBug, but aren't at the moment:

• A lot of panic!()s in the VM should probably be converted to TvixBug.
• Serialization code (JSON, XML) seems to use their respective unserializeable error variant for internal values that should probably be TvixBug, as you should never be able to serialize internal values from user code.