This commit adds RVALUE_OVERHEAD for storing metadata at the end of the
slot. This commit moves the ractor_belonging_id in debug builds from the
flags to RVALUE_OVERHEAD which frees the 16 bits in the headers for
object shapes.

Prior to this commit it was possible to call `ObjectSpace._id2ref` with
an offset static symbol object_id and get back a new, incorrectly tagged
symbol:

```
> sensible_sym = ObjectSpace._id2ref(:a.object_id)
=> :a
> nonsense_sym = ObjectSpace._id2ref(:a.object_id + 40)
=> :a
> sensible_sym == nonsense_sym
=> false
```

`nonsense_sym` ends up tagged with `RUBY_ID_INSTANCE` instead of
`RB_ID_LOCAL`. That means we can do silly things like:

```
> foo = Object.new
> foo.instance_variable_set(:a, 123)
(irb):2:in `instance_variable_set': `a' is not allowed as an instance variable name (NameError)
> foo.instance_variable_set(ObjectSpace._id2ref(:a.object_id + 40), 123)
=> 123
> foo.instance_variables
=> [:a]
```

This was happening because `get_id_entry` ignores the tag bits when
looking up the symbol. So `rb_id2str(symid)` would return a value and
then we'd continue on with the nonsense `symid`.

This commit prevents the situation by checking that the `symid` actually
matches what we get back from `get_id_entry`. Now we get a `RangeError`
for the nonsense id:

```
> ObjectSpace._id2ref(:a.object_id)
=> :a
> ObjectSpace._id2ref(:a.object_id + 40)
(irb):1:in `_id2ref': 0x000000000013f408 is not symbol id value (RangeError)
```

Co-authored-by: John Hawthorn <jhawthorn@github.com>

The test disables GC but never reenables it. Before this patch, running
all tests would have a peak RSS in the main process of >4GB. After this
patch, peak RSS in the main process is <500MB.

This fixes cases where exceptions raised using Thread#raise are
swallowed by finalizers and not delivered to the running thread.

This could cause issues with finalizers that rely on pending interrupts,
but that case is expected to be rarer.

Fixes [Bug #13876]
Fixes [Bug #15507]

Co-authored-by: Koichi Sasada <ko1@atdot.net>

Ensure that the argument is an Integer or implicitly convert to,
before dereferencing as a Bignum.  Addressed a regression in
b99833baec2.

Reported by u75615 at https://hackerone.com/reports/898614

objspace->id_to_obj_tbl can contain died objects because of lazy
sweep, so that it should check liveness.

It has caused CI failures.

* d0cd0866d82a58933e5dccd073c753c0c2ad4eb5

  Disable GC during rb_objspace_reachable_object_p

* 89cef1c56b3a0f9c5e6ccc22a5044477a4fd16c1

  Version guard for [Feature #15974]

* 796eeb6339952d92ae1b353d450c7883e589852d.

  Fix up [Feature #15974]

* 928260c2a613bbdd4402c300e0bf86ae7562e52a.

  Warn in verbose mode on defining a finalizer that captures the object