Attempt S4 dispatch for intersect() and union(), maybe - alternative

[hcirellu]

Here I use setMethod for S3 classes to achieve a S4 dispatch. With this approach the regular S4 dispatch takes place and no consideration is necessary within our functions/methods.
Let's see if this is better than PR #305 or not.

[hcirellu]

Of course, documentation has to be added, but other than that we don't have to select S4 methods ourselves. WDYT?

[MichaelChirico]

In {nanotime}, they leave this unspecified and therefore it becomes base::intersect().

Overall I might expect we remove this from methods:

if (!(is.integer64(x) || is.integer64(y))) return(base::...)

In favor of S4 dispatch handling that part. Then we only need to define for signature(x="integer64", y="ANY") and signature(x="ANY", y="integer64").

Also, is there any residual benefit of the S3 method once we add the S4 dispatch?

[hcirellu]

Please have a look at the man page, because of the additional S4 method entries. Your approach in PR #305 does not have those, which is cleaner in an overview, but hides the added S4 dispatch capabilities of the set ops.