Make pointer size consistent between backends

[ankane]

This PR makes the pointer size consistent between backends. Follow up to #179.

require "fiddle"

ptr = Fiddle::Pointer.new(0)
p ptr.size
p ptr.ref.size

ptr = Fiddle::Pointer["hello world"]
ptr.size = 0
p ptr.to_s
p ptr.to_str

JRuby + TruffleRuby before

9223372036854775807
8
""
""

CRuby + JRuby + TruffleRuby after

0
0
"hello world"
""

[kou]

Thanks.

[ankane]

Thank you

[eregon]

Looking at this this, why does ptr.size = 0 have no effect here, but it has on ptr.to_str in the previous test?
Shouldn't ptr.size = 0 change the @size of ptr?

[ankane]

This is the current behavior of CRuby, so the PR mirrors it.