The return value of Module#set_temporary_name was changed
to return `self`, but the existing tests
did not verify this.

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]

The "EXIVAR" terminology has been replaced by "gen fields"
AKA "generic fields".

Exivar implies variable, but generic fields include more than
just variables, e.g. `object_id`.

I don't think this ever happened, but we should raise the same error
we'd raise on lookup

We rely on the GC to clear this when the GC is run on another EC than
the cache.