BREAKING for the very small minority of folks that use `nippy/stress-data`.
Changes:
1. Make `nippy/stress-data` a function
It's unnecessarily wasteful to generate and store all this data when it's not
being used in the common case.
2. Make data deterministic
The stress data will now generally be stable by default between different versions
of Nippy, etc. This will help support an upcoming test for stable serialized output.
