feat(util/rb_tree): add find_if method
feat(util/rb_tree): add find_if method

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