sstring: fix to_sstring() for all sstring types
sstring: fix to_sstring() for all sstring types

sstring is a family of types (basic_string<...>), make to_sstring work for
all of them.

shaunstanislaus/scylla,raphaelsc/seastar,dwdm/seastar,dreamsxin/seastar,asias/scylla,raphaelsc/scylla,jonathanleang/seastar,acbellini/seastar,tempbottle/scylla,jonathanleang/seastar,mixja/seastar,linearregression/seastar,chunshengster/seastar,kangkot/scylla,bzero/seastar,scylladb/seastar,shaunstanislaus/scylla,avikivity/scylla,rentongzhang/scylla,kjniemi/scylla,victorbriz/scylla,scylladb/scylla,ducthangho/imdb,guiquanz/scylla,leejir/seastar,kjniemi/scylla,hongliangzhao/seastar,linearregression/scylla,guiquanz/scylla,glommer/scylla,jonathanleang/seastar,capturePointer/scylla,eklitzke/scylla,dwdm/seastar,joerg84/seastar,bzero/seastar,bowlofstew/scylla,stamhe/seastar,hongliangzhao/seastar,chunshengster/seastar,dwdm/scylla,linearregression/scylla,shaunstanislaus/scylla,raphaelsc/seastar,phonkee/scylla,kjniemi/scylla,scylladb/scylla,avikivity/seastar,xtao/seastar,aruanruan/scylla,scylladb/scylla-seastar,tempbottle/scylla,duarten/scylla,tempbottle/seastar,eklitzke/scylla,avikivity/scylla,bowlofstew/seastar,rluta/scylla,mixja/seastar,gwicke/scylla,wildinto/scylla,wildinto/scylla,justintung/scylla,slivne/seastar,norcimo5/seastar,bowlofstew/seastar,scylladb/scylla,dwdm/scylla,slivne/seastar,syuu1228/seastar,gwicke/scylla,koolhazz/seastar,respu/scylla,shyamalschandra/seastar,flashbuckets/seastar,duarten/scylla,sjperkins/seastar,scylladb/scylla-seastar,bowlofstew/seastar,cloudius-systems/seastar,dwdm/seastar,acbellini/scylla,flashbuckets/seastar,joerg84/seastar,kjniemi/seastar,rluta/scylla,syuu1228/seastar,scylladb/seastar,xtao/seastar,cloudius-systems/seastar,anzihenry/seastar,raphaelsc/scylla,kjniemi/seastar,gwicke/scylla,shyamalschandra/seastar,asias/scylla,rluta/scylla,norcimo5/seastar,victorbriz/scylla,ducthangho/imdb,phonkee/scylla,asias/scylla,kangkot/scylla,rentongzhang/scylla,capturePointer/scylla,victorbriz/scylla,acbellini/scylla,bowlofstew/scylla,linearregression/seastar,phonkee/scylla,respu/scylla,kjniemi/seastar,bowlofstew/scylla,norcimo5/seastar,sjperkins/seastar,ducthangho/imdb,raphaelsc/scylla,cloudius-systems/seastar,senseb/scylla,stamhe/scylla,koolhazz/seastar,printedheart/seastar,guiquanz/scylla,stamhe/seastar,sjperkins/seastar,xtao/seastar,justintung/scylla,anzihenry/seastar,raphaelsc/seastar,koolhazz/seastar,stamhe/seastar,linearregression/seastar,tempbottle/seastar,aruanruan/scylla,duarten/scylla,joerg84/seastar,printedheart/seastar,kangkot/scylla,wildinto/seastar,wildinto/seastar,dreamsxin/seastar,aruanruan/scylla,avikivity/seastar,syuu1228/seastar,acbellini/scylla,eklitzke/scylla,tempbottle/seastar,bzero/seastar,justintung/scylla,wildinto/scylla,leejir/seastar,wildinto/seastar,respu/scylla,mixja/seastar,senseb/scylla,stamhe/scylla,rentongzhang/scylla,leejir/seastar,avikivity/seastar,hongliangzhao/seastar,senseb/scylla,tempbottle/scylla,flashbuckets/seastar,printedheart/seastar,acbellini/seastar,dreamsxin/seastar,stamhe/scylla,dwdm/scylla,linearregression/scylla,chunshengster/seastar,acbellini/seastar,glommer/scylla,avikivity/scylla,scylladb/scylla,glommer/scylla,anzihenry/seastar,scylladb/seastar,scylladb/scylla-seastar,capturePointer/scylla,shyamalschandra/seastar,slivne/seastar