Automated testing of TimeSteward internals

[elidupree]

Our main tool for testing TimeSteward behavior should be cross verified time stewards. If two time stewards receive the same valid input and give different results, there is an internal error. However, this testing should be cautious not to give false-positives when the caller gives invalid input.

I began implementing something like this, but ran into trouble with Rust polymorphism limitations. In the short term, working around them may require a whole pile of macros.

We can also make a wrapper class that tests whether a TimeSteward obeys the valid_since() rules.

[elidupree]

The basic implementation of cross verified time stewards is now working. There are many more features that can be added, though.

[elidupree]

I have created a pretty good crossverified test case. More work could still be done in this area, but I consider this sufficient for MVP.