Add ReferenceStorage for manual allocation of references

[HertzDevil]

This adds the Instance type mentioned in #13481. The naming is not final.

[straight-shoota]

LGTM.

As mentioned in the OP, naming is still kind of preliminary. But in absence of any really clever idea, we can just take ReferenceStorage.
In case there is something inherently wrong about it, we can still change it as it's marked as experimental.

[HertzDevil]

What about the minimal implementations for Object's abstract defs? Do you think they make sense?

[straight-shoota]

Not sure... Maybe we could leave them out for now and add them later if we notice they're missing? It's easy to patch them in ad-hoc, so this isn't holding up anything.

I suppose instances of ReferenceStorage wouldn't really be used on their own. The most typical use case we have been thinking of (so far):

just_some_variable_to_put_this_on_the_stack = uninitialized ReferenceStorage(Foo)
foo = Foo.pre_initialize(pointerof(just_some_variable_to_put_this_on_the_stack))

And just_some_variable_to_put_this_on_the_stack is never used again (unless you want to allocate a something else in that storage, but that would be the same usage as the second line here).

Probably, this pattern will be abstracted away in a macro for convenience, further reducing direct usage of ReferenceStorage.

[HertzDevil]

The problem is #==, #to_s and #hash cannot be left out at all because the type inherits directly from Value rather than Struct. (I guess you could leave out #== because Value has a catch-all def that returns false)

[straight-shoota]

Sounds good. Let's keep it then 👍

[straight-shoota]

This was eventually reverted in #14207 for 1.11.1 due to #14194. An alternative impementation is available at #14204.