feat(util/sexpr/format): small change
feat(util/sexpr/format): small change

The idea is to avoid unnecessary memory allocations.
It may also help with the lean.js problem we are fighting with.

leanprover/lean,fpvandoorn/lean,avigad/lean,fgdorais/lean,c-cube/lean,avigad/lean,johoelzl/lean,leodemoura/lean,javra/lean,Kha/lean,eigengrau/lean,leanprover/lean,fpvandoorn/lean,htzh/lean,javra/lean,rlewis1988/lean,johoelzl/lean,digama0/lean,htzh/lean,leanprover/lean,fpvandoorn/lean2,digama0/lean,sp3ctum/lean,soonhokong/lean-windows,leanprover-community/lean,digama0/lean,levnach/lean,fpvandoorn/lean2,digama0/lean,leanprover-community/lean,soonhokong/lean,fpvandoorn/lean2,leanprover/lean,rlewis1988/lean,avigad/lean,rlewis1988/lean,soonhokong/lean-windows,javra/lean,johoelzl/lean,c-cube/lean,soonhokong/lean,eigengrau/lean,eigengrau/lean,Kha/lean,johoelzl/lean,sp3ctum/lean,leanprover/lean,dselsam/lean,fgdorais/lean,leanprover-community/lean,avigad/lean,levnach/lean,leodemoura/lean,soonhokong/lean-windows,fpvandoorn/lean2,c-cube/lean,Kha/lean,leanprover-community/lean,fgdorais/lean,avigad/lean,sp3ctum/lean,fpvandoorn/lean,Kha/lean,digama0/lean,Kha/lean,c-cube/lean,dselsam/lean,UlrikBuchholtz/lean,soonhokong/lean-osx,soonhokong/lean,rlewis1988/lean,leodemoura/lean,fgdorais/lean,leodemoura/lean,eigengrau/lean,fpvandoorn/lean,johoelzl/lean,UlrikBuchholtz/lean,htzh/lean,digama0/lean,htzh/lean,fgdorais/lean,rlewis1988/lean,fpvandoorn/lean2,soonhokong/lean-windows,javra/lean,levnach/lean,leodemoura/lean,htzh/lean,leodemoura/lean,UlrikBuchholtz/lean,soonhokong/lean-windows,soonhokong/lean,UlrikBuchholtz/lean,soonhokong/lean-osx,fpvandoorn/lean,sp3ctum/lean,dselsam/lean,avigad/lean,soonhokong/lean,levnach/lean,soonhokong/lean-osx,Kha/lean,dselsam/lean,leanprover/lean,levnach/lean,soonhokong/lean-osx,dselsam/lean,eigengrau/lean,fpvandoorn/lean,johoelzl/lean,leanprover-community/lean,javra/lean,rlewis1988/lean,leanprover-community/lean,c-cube/lean,dselsam/lean,soonhokong/lean-osx,UlrikBuchholtz/lean,sp3ctum/lean,fgdorais/lean