Before this patch, Ractor::IsolationError reported an incorrect constant
path when constant was found through `rb_const_get_0()`.

In this code, Ractor::IsolationError reported illegal access against
`M::TOPLEVEL`, where it should be `Object::TOPLEVEL`.

```ruby
TOPLEVEL = [1]

module M
  def self.f
    TOPLEVEL
  end
end

Ractor.new { M.f }.value
```

This was because `rb_const_get_0()` built the "path" part referring to
the module/class passed to it in the first place. When a constant was
found through recursive search upwards, the module/class which the
constant was found should be reported.

This patch fixes this issue by modifying rb_const_search() to take a
VALUE pointer to be filled with the module/class where the constant was
found.

[Bug #21782]

Although the Ractor API is still experimental and may change, and there
may be some implementation issues, we should no longer say that there
are many.

Hopefully we can remove this warning entirely for Ruby 4.1

This refactors the concurrent set to examine and reserve a slot via CAS
with the hash, before then doing the same with the key.

This allows us to use an extra bit from the hash as a "continuation bit"
which marks whether we have ever probed past this key while inserting.
When that bit isn't set on deletion we can clear the field instead of
placing a tombstone.