Add `CCSeq.exists` and `CCSeq.for_all`

[bbc2]

Would you be interested in adding exists and for_all to CCSeq or are they considered outside of the scope of this package and better covered by oseq?

I'm asking because I find them useful and because CCSeq already has exists2 and for_all2.

[c-cube]

That sounds reasonable indeed, I thought they were in the standard Seq but apparently not.