feat(library/kernel_bindings): constraint Lua API

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(util/list_lua): typo

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
feat(util/list_lua): add length method to list Lua API

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
test(lua): add constraint API tests, and fix minor bugs

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
chore(travis): use lean-build@googlegroups