A set of small improvements for object_id, inspired by Ruby 4.0 object_id optimizations and with an eye for future removal of _id2ref.

Ideas to try:

• Reduce reads of volatile fields in lazy variable accessors.
• Use the smallest java.lang.Number box for a given ID.
• Propagate ThreadContext to avoid getting runtime the slow way.
• Use long rather than IRubyObject in MRIRecursionGuard. (rejected as this is unused, deprecated code and the replacement does not use object_id)
• Cache RubyInteger version after access, to avoid constructing it repeatedly. (rejected for now; storing RubyFixnum would negate size optimizations above and an alternative box requires more thought)