feat(kernel/expr): use the expression depth in the hash code

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
fix(kernel/module): deadlock

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
test(lua): new module system test

Signed-off-by: Leonardo de Moura <leonardo@microsoft.com>
test(lua): new module system test