Fix: revert change to keep bytesize in stack

src/fiber/stack.cr

@@ -2,22 +2,14 @@ class Fiber
   # :nodoc:
   struct Stack
     getter pointer : Void*
-    getter bytesize : Int32
+    getter bottom : Void*
     getter? reusable : Bool
 
-    def initialize(@pointer, bottom : Void*, *, @reusable = false)
-      @bytesize = (bottom - @pointer).to_i32
-    end
-
-    def initialize(@pointer, @bytesize, *, @reusable = false)
-    end
-
-    def bottom : Void*
-      @pointer + @bytesize
+    def initialize(@pointer, @bottom, *, @reusable = false)
     end
 
     def first_addressable_pointer : Void**
-      ptr = bottom                              # stacks grow down
+      ptr = @bottom                             # stacks grow down
       ptr -= sizeof(Void*)                      # point to first addressable pointer
       Pointer(Void*).new(ptr.address & ~15_u64) # align to 16 bytes
     end

src/fiber/stack_pool.cr

@@ -17,7 +17,7 @@ class Fiber
 
     def finalize
       @deque.each do |stack|
-        Crystal::System::Fiber.free_stack(stack.pointer, stack.bytesize)
+        Crystal::System::Fiber.free_stack(stack.pointer, STACK_SIZE)
       end
     end
 
@@ -26,7 +26,7 @@ class Fiber
     def collect(count = lazy_size // 2) : Nil
       count.times do
         if stack = @deque.shift?
-          Crystal::System::Fiber.free_stack(stack.pointer, stack.bytesize)
+          Crystal::System::Fiber.free_stack(stack.pointer, STACK_SIZE)
         else
           return
         end
@@ -43,11 +43,11 @@ class Fiber
     # Removes a stack from the bottom of the pool, or allocates a new one.
     def checkout : Stack
       if stack = @deque.pop?
-        Crystal::System::Fiber.reset_stack(stack.pointer, stack.bytesize, @protect)
+        Crystal::System::Fiber.reset_stack(stack.pointer, STACK_SIZE, @protect)
         stack
       else
         pointer = Crystal::System::Fiber.allocate_stack(STACK_SIZE, @protect)
-        Stack.new(pointer, STACK_SIZE, reusable: true)
+        Stack.new(pointer, pointer + STACK_SIZE, reusable: true)
       end
     end