feat(kernel/type_checker): add swap method
feat(kernel/type_checker): add swap method

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

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