feat(frontends/lean): add parallel_import option
feat(frontends/lean): add parallel_import option

Signed-off-by: Leonardo de Moura <7610bae85f2b530654cc716772f1fe653373e892@microsoft.com>

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