Need an assertion policy

Currently, we use `debug_assert` in most cases because we check the assertions in tests and we don't want to check them again in the release binary.

But it's not sustainable. Bugs will occur for any reason. It seems better to use `assert` to crash early because we have a plan to add [a bug tracker](https://github.com/CodeChain-io/codechain/issues/196).

I think we need a guideline to decide when to use `assert` or `debug_assert`.