Improve Box of reference types

[waj]

Right now Box(T) always allocates memory to create a box for any object. With these changes it only allocates memory for value types.

For example:

Box.box(123)   # => Pointer(Void)@0x104a29ff0
Box.box(nil)   # => Pointer(Void).null

foo = Foo.new  # => #<Foo:0x10b1b7ff0>
Box.box(foo)   # => Pointer(Void)@0x10b1b7ff0