Make ptr public

[hommeabeil]

Having access to the pointer in the different struct could be really handy. Some time it would be useful to simply use the fdisk-sys crate when the wrapper does not offer the function. However, all the pointer are public but within the crate. Does making the pointer public could break something ?

[nikita-dubrovskii]

I would suggest to add required functionality to the the crate, otherwise user may do wrong things with the pointer.