perf(library/unifier): improve flex_rigid performance
perf(library/unifier): improve flex_rigid performance

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

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