feat (library/theories/topology/basic) : add separation theorems
feat (library/theories/topology/basic) : add separation theorems

add T0, T1, T2 separation theorems and add closed singleton theorem for T1 spaces
johoelzl/lean,soonhokong/lean-osx,UlrikBuchholtz/lean,rlewis1988/lean,fpvandoorn/lean2,fpvandoorn/lean2,Kha/lean,avigad/lean,fpvandoorn/lean,johoelzl/lean,leanprover-community/lean,rlewis1988/lean,leanprover/lean,avigad/lean,leanprover-community/lean,dselsam/lean,soonhokong/lean-osx,leodemoura/lean,leodemoura/lean,digama0/lean,avigad/lean,fgdorais/lean,johoelzl/lean,soonhokong/lean,javra/lean,soonhokong/lean-osx,rlewis1988/lean,fpvandoorn/lean2,fpvandoorn/lean2,Kha/lean,leanprover-community/lean,fpvandoorn/lean,fpvandoorn/lean,johoelzl/lean,soonhokong/lean-windows,fpvandoorn/lean2,digama0/lean,javra/lean,dselsam/lean,levnach/lean,leodemoura/lean,fpvandoorn/lean,leanprover/lean,fgdorais/lean,digama0/lean,avigad/lean,soonhokong/lean,leanprover-community/lean,Kha/lean,fgdorais/lean,fgdorais/lean,dselsam/lean,digama0/lean,levnach/lean,johoelzl/lean,rlewis1988/lean,fpvandoorn/lean,digama0/lean,javra/lean,digama0/lean,soonhokong/lean-osx,johoelzl/lean,leodemoura/lean,leanprover-community/lean,leanprover/lean,soonhokong/lean,leanprover/lean,rlewis1988/lean,levnach/lean,dselsam/lean,fpvandoorn/lean,UlrikBuchholtz/lean,dselsam/lean,leanprover-community/lean,javra/lean,soonhokong/lean-windows,soonhokong/lean,Kha/lean,avigad/lean,UlrikBuchholtz/lean,avigad/lean,UlrikBuchholtz/lean,leodemoura/lean,rlewis1988/lean,levnach/lean,soonhokong/lean-windows,levnach/lean,dselsam/lean,leanprover/lean,UlrikBuchholtz/lean,soonhokong/lean-osx,soonhokong/lean-windows,leanprover/lean,leodemoura/lean,javra/lean,Kha/lean,fgdorais/lean,soonhokong/lean,fgdorais/lean,Kha/lean,soonhokong/lean-windows