`Fiddle::Pointer#to_value` is unsafe

Well, it's pretty self-explanatory. It returns (VALUE)(data->ptr), and ptr is obviously not marked, so can be GCed. This method is either should be removed completely (it'll still e possible to hack around with Fiddle.dlunwrap, but this at least feels hacky) or to be allowed only for pointers marked to be in fact VALUEs (mark return value type in Function constructor), and they of course should gc_mark their ptrs. Btw, there is no way to create a pointer from Ruby object, either.

Comienza leyendo la implementación de Fiddle::Pointer#to_value y del constructor de Function; después, inspecciona cómo interactúan el marcado de punteros y Fiddle.dlunwrap con el recolector de basura de Ruby. El issue presenta alternativas de eliminación y restricción de tipos; para darlo por terminado se requiere un diseño de seguridad decidido, una implementación y cobertura del comportamiento afectado.