Add convinience methods to get path to memfd on the filesystem

[AlexanderSchuetz97]

Hello,

I require the path of the Memfd on disk for my use case.
I.e. a fn that returns "/proc/pid_here/fd/fd_number_here"

In my opinion a utility function to get this path would not be out of place in the Memfd struct.
Since its not much effort I have a submitted a PR to do just that.

See #68

Sincerely
Alexander Schütz