feat: allow taking pointerof(self) in structs

[BlobCodes]

This MR allows taking pointerof(self) inside a struct instance.
It only works for structs, there's an error message when attempting to take pointerof(self) inside a class or meta-class since it would be hard to define proper behavior for them.

This is already possible by defining a empty ivar at the beginning of the struct, this just makes it simpler and safer:

struct Foo
  # Before
  @beginning = uninitialized UInt8[0]
  def this : self*
    pointerof(@beginning).as(self*)
  end

  # After
  def this : self*
    pointerof(self)
  end
end

I think this MR might be a bit controversial, but since the change was so simple, I thought I'd just create the MR directly.

TODO:

• Add specs
• Test with interpreter

[straight-shoota]

Sounds reasonable.
Still, I think such a change would require some discussion. What is it useful for? Why is this a good solution for that? What are the drawbacks? Alternatives?
This may sound a bit bureaucratic, but I think it would be best to start an RFC about this: https://github.com/crystal-lang/rfcs/
It doesn't need to be big. A very concise description should be fine.
Since this is a language change, it's very useful to have it properly documented.

[ysbaddaden]

There would be a couple benefit:

1. the intent would be explicit;

2. as far as I know we make no guarantee over the order of ivars in objects (be they structs or classes), and only guarantee that order ivars are in the order of definition for extern structs (lib structs or @[Extern]) that only accept a limited set of primitive types.

   When we implement a mechanism to order ivars, to reduce padding for example, taking the pointer of the first defined ivar won't necessarily be the first struct member (unless extern struct), while pointerof(self) would.

Though I'm wondering if self isn't already a pointer type, so instead of taking a pointer to self, we shouldn't just cast it to a pointer (as in references) 🤔

[ysbaddaden]

I thought I had some use cases with structs that are directly mapped in memory but I just replaced them with an empty static array: @payload = uninitialized UInt8[0] then @payload.to_unsafe 🤷