Adapt pointer to closure

[idegtiarenko]

When implementing an integration with the libfuse I noticed that the readdir callback supplies the user with a fuse_fill_dir_t that is a pointer to a function that supposed to be called for every child item in the directory.

I have not found a way to automatically nor programmatically convert that pointer to a closure so that it could be called within java.
I believe others implementations might have a similar issue that required a split package to fix.

This change adds a similar implementation to the ClosureManager/NativeClosureManager so that the pointer could be adapted to the interface callable from java.

I am new to the jnr-ffi lib. Please let me know if I miss anything or if it is a right way to resolve this issue.

Thanks!