R: update_vs_ref checks if a graph object was supplied

Because it might be NULL if we are dealing with detached vs/es.
R: function to get uuid of a graph
R: make union, intersection, difference S3 generics
R: utility function drop_null
R: better handling of data type versioning
R: operations on vertex/edge sequences:

- concatenation
- union
- intersection
- difference
- reverse
- unique