`Box.box` does not need to allocate for values smaller than a pointer

[straight-shoota]

Box.box can cast values of small primitives types to pointer and pass that around directly, instead of allocating space and passing the pointer to that memory.

Extracted from #15562 (comment)

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