atomic_cell: add collection support
atomic_cell: add collection support

We leave interpretation to the backing type (map/set/list), so there is
not much code here.

justintung/scylla,victorbriz/scylla,bowlofstew/scylla,scylladb/scylla,kjniemi/scylla,acbellini/scylla,senseb/scylla,bowlofstew/scylla,wildinto/scylla,eklitzke/scylla,dwdm/scylla,aruanruan/scylla,glommer/scylla,shaunstanislaus/scylla,kangkot/scylla,linearregression/scylla,shaunstanislaus/scylla,gwicke/scylla,tempbottle/scylla,justintung/scylla,respu/scylla,acbellini/scylla,rentongzhang/scylla,stamhe/scylla,scylladb/scylla,aruanruan/scylla,rentongzhang/scylla,scylladb/scylla,raphaelsc/scylla,rentongzhang/scylla,rluta/scylla,capturePointer/scylla,stamhe/scylla,avikivity/scylla,duarten/scylla,gwicke/scylla,justintung/scylla,linearregression/scylla,rluta/scylla,glommer/scylla,phonkee/scylla,eklitzke/scylla,raphaelsc/scylla,respu/scylla,tempbottle/scylla,asias/scylla,linearregression/scylla,phonkee/scylla,guiquanz/scylla,tempbottle/scylla,kangkot/scylla,dwdm/scylla,scylladb/scylla,capturePointer/scylla,acbellini/scylla,wildinto/scylla,eklitzke/scylla,glommer/scylla,stamhe/scylla,senseb/scylla,dwdm/scylla,gwicke/scylla,guiquanz/scylla,bowlofstew/scylla,asias/scylla,rluta/scylla,kjniemi/scylla,victorbriz/scylla,duarten/scylla,respu/scylla,asias/scylla,aruanruan/scylla,duarten/scylla,phonkee/scylla,victorbriz/scylla,guiquanz/scylla,avikivity/scylla,raphaelsc/scylla,kangkot/scylla,senseb/scylla,kjniemi/scylla,avikivity/scylla,wildinto/scylla,shaunstanislaus/scylla,capturePointer/scylla