fix(frontends/lean/elaborator): instantiate assigned metavariables before collecting unassigned ones
fix(frontends/lean/elaborator): instantiate assigned metavariables before collecting unassigned ones

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

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