Before this change, GC'ing any Ractor object caused you to lose all
enabled tracepoints across all ractors (even main). Now tracepoints are
ractor-local and this doesn't happen. Internal events are still global.

Fixes [Bug #19112]

This reverts commit 26a9e0b4e31f7b5a9cbd755e0a15823a8fa51bae partially.

to adopt strict shareable rule.

* (basically) shareable objects only refer shareable objects
* (exception) shareable objects can refere unshareable objects
  but should not leak reference to unshareable objects to Ruby world

When we clone a complex imemo_fields, it calls creates the imemo_fields
using rb_imemo_fields_new_complex, which allocates and initializes a new
st_table. However, st_replace will directly replace any exisiting fields
in the st_table, causing it to leak.

For example, this script demonstrates the leak:

    obj = Class.new
    8.times do |i|
      obj.instance_variable_set(:"@test#{i}", nil)
      obj.remove_instance_variable(:"@test#{i}")
    end

    obj.instance_variable_set(:"@test", 1)

    10.times do
      100_000.times do
        obj.dup
      end

      puts `ps -o rss= -p #{$$}`
    end

Before:

    26320
    39296
    52320
    63136
    75520
    87008
    97856
    114800
    120864
    133504

After:

    16288
    20112
    20416
    20720
    20800
    20864
    21184
    21424
    21904
    21904

The imemo_fields_new function takes a capacity in the number of fields to
preallocate. rb_imemo_fields_new_complex_tbl is using it incorrectly because
it is preallocating sizeof(struct rb_fields) number of fields.

We should not assume that a complex imemo_field takes only one additional
VALUE space. This is fragile as it will break if we add additional fields
to complex imemo_field.

These used to be used by the parser