Fix Box(Pointer).box to not allocate pointer storage on the heap

[ysbaddaden]

While discussing to use Box(T) in #15395 I noticed that Box(Pointer).box(ptr) allocates into the GC heap to store the pointer address into.

That felt like an overlook since we don't need to allocate to box a Reference or Nil, so it should be fine to box a raw pointer without allocating an intermediary pointer. Am I missing something?

I only allowed T < Pointer in addition to the Reference and nilable Reference cases. I'm not sure a Pointer appearing in an union along Nil or Reference would behave correctly —is it passing the {type, value} union by value?

[straight-shoota]

Pointer | Nil and Pointer | Reference are mixed union types, so they cannot be represented by just a pointer. We need to allocate on the heap. They are not very common types, thus it doesn't matter much.
We might consider adding specs for these types though, to make sure the behaviour is correct (can be in this PR or a followup).

[HertzDevil]

If this is allowed, then perhaps values smaller than a pointer also don't need an allocation either? Primitive numbers for example:

struct Box(T)
  def self.box(object : T) : Void*
    {% if T <= Int::Primitive && sizeof(T) <= sizeof(Void*) %}
      Pointer(Void).new(object.to_u64!)
    {% end %}
  end

  def self.unbox(pointer : Void*) : T
    {% if T <= Int::Primitive && sizeof(T) <= sizeof(Void*) %}
      T.new!(pointer.address)
    {% end %}
  end
end

[ysbaddaden]

Since the pointer returned by .box can only safely be used with .unbox(ptr) then I guess it would be fine 🤔