Difference with `Fiddle::Pointer#ref` and FFI backend

[ankane]

Hi, with the C backend, the code below updates ptr in-place. However, with the FFI backend, ptr is unchanged (with both JRuby and TruffleRuby).

require "fiddle"

ptr = Fiddle::Pointer["hello"]
i = ptr.to_i

ptr.ref[0, Fiddle::SIZEOF_VOIDP] = Fiddle::Pointer["world"]
p ptr.to_i == i # false for C backend, true for FFI backend

I'm having trouble finding the cause (to try to make the behavior consistent), so wanted to see if anyone had ideas.

[kou]

In general, the C backend is the referenced implementation. So we should fix the FFI backend.

[ankane]

Submitted a PR in #185.