Implement an internal stub method to make time travel not dependent on mocha

Before this patch time travel feature depends on mocha's stub feature.
Since mocha is not a Rails dependency we can't depend on this specific
library.

We choose to use our own implementation to avoid adding one more
dependency to Rails.

Fixes #13380.

I'm wondering if we should include these internal stubs method only in Date and Time classes or if we should move this feature to another file to make it available to our users.

cc @dhh @senny @tenderlove