Make CCPersistentHashtbl.S.merge more general.

[johanneskloos]

This patch brings the merge function of CCPersistentHashtbl
in line with the merge functions of other maps
(Map, BatMap from batteries, CCWBTree). In particular, its signature
changes from the restrictive
merge: (key -> 'a option -> 'a option -> 'a option) -> 'a t -> 'a t -> 'a t
to a more general
merge: (key -> 'a option -> 'b option -> 'c option) -> 'a t -> 'b t -> 'c t

[c-cube]

Indeed, that's a good idea. Thanks for the patch, would you mind adding your name to AUTHORS.adoc before I merge?

[c-cube]

Thanks!

[rgrinberg]

Just a heads up, I think that core's interface is slightly stronger for this particular function. Rather than having 'a option -> 'b option core does

[ `Left of 'a
| `Right of 'b
| `Both of 'a * 'b ]

This is slightly superior because the two optional values being None is impossible (if I've interpreted what this function does correctly)

[c-cube]

Oh yes, I remember having seen that on #OCaml recently. That would be actually nice… But then we would need to have the same in CCMap (which would be a good thing).