feat(frontends/lean/parser): allow tactic to be used to fill holes in definitions
feat(frontends/lean/parser): allow tactic to be used to fill holes in definitions

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

digama0/lean,digama0/lean,leanprover/lean,htzh/lean,leanprover-community/lean,leanprover/lean,codyroux/lean0.1,leanprover-community/lean,leodemoura/lean,dselsam/lean,soonhokong/lean-osx,Kha/lean,fgdorais/lean,codyroux/lean0.1,soonhokong/travis_test,leanprover-community/lean,UlrikBuchholtz/lean,avigad/lean,fgdorais/lean,Kha/lean,fpvandoorn/lean,soonhokong/lean,rlewis1988/lean,dselsam/lean,johoelzl/lean,soonhokong/lean-windows,digama0/lean,fgdorais/lean,soonhokong/travis_test,sp3ctum/lean,javra/lean,UlrikBuchholtz/lean,johoelzl/lean,codyroux/lean0.1,dselsam/lean,UlrikBuchholtz/lean,soonhokong/lean,rlewis1988/lean,soonhokong/travis_test,avigad/lean,eigengrau/lean,htzh/lean,fpvandoorn/lean2,levnach/lean,leanprover/lean,johoelzl/lean,c-cube/lean,avigad/lean,javra/lean,fpvandoorn/lean2,dselsam/lean,soonhokong/lean-osx,fpvandoorn/lean,leodemoura/lean,levnach/lean,leanprover/lean,levnach/lean,sp3ctum/lean,htzh/lean,digama0/lean,rlewis1988/lean,UlrikBuchholtz/lean,codyroux/lean0.1,htzh/lean,soonhokong/lean,rlewis1988/lean,c-cube/lean,sp3ctum/lean,johoelzl/lean,avigad/lean,Kha/lean,leodemoura/lean,fgdorais/lean,fgdorais/lean,leanprover/lean,johoelzl/lean,soonhokong/lean-windows,johoelzl/lean,leanprover-community/lean,soonhokong/lean-osx,fpvandoorn/lean2,soonhokong/travis_test,fpvandoorn/lean2,digama0/lean,fpvandoorn/lean,levnach/lean,fpvandoorn/lean2,eigengrau/lean,c-cube/lean,fpvandoorn/lean,Kha/lean,leanprover-community/lean,soonhokong/lean-windows,soonhokong/lean,leanprover-community/lean,leodemoura/lean,levnach/lean,c-cube/lean,soonhokong/lean-osx,javra/lean,soonhokong/lean-windows,UlrikBuchholtz/lean,sp3ctum/lean,sp3ctum/lean,leanprover/lean,javra/lean,javra/lean,fpvandoorn/lean,fgdorais/lean,Kha/lean,rlewis1988/lean,soonhokong/lean-windows,digama0/lean,soonhokong/lean-osx,dselsam/lean,dselsam/lean,soonhokong/lean,leodemoura/lean,htzh/lean,eigengrau/lean,eigengrau/lean,Kha/lean,rlewis1988/lean,leodemoura/lean,fpvandoorn/lean,avigad/lean,avigad/lean,c-cube/lean,eigengrau/lean