Avoid 'Too late to run CHECK block' warning when loaded in an eval

Yes, could have used "no warning 'void'", but since we are already
using XS...
Turn callback parameters into objects

Used to pass the root/start ops for eval "" and the main
code segment, which have the root/start field empty and the
corresponding values stored in PL_{main,eval}_{root,start}.
Add a 'no nextstate' example pragma