feat(library/expr_lt): add lex comparison for expression pairs
feat(library/expr_lt): add lex comparison for expression pairs

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

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