Fix type checker for let expressions
Fix type checker for let expressions

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

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