efyj: add a --stress mode

The stress mode compares solver and caches on a random options lists.

For example:

$ efyj -s 100 Car.dxi
- run stress mode for 100 random run
Processing tests/Car.dxi
Time elapsed to:
- read model........ : 0.000452362s
- get response for random 100 options:
  - basic solver.............. : 8.5067e-05s
  - hash cache................ : 0.00240485s
  - hash cache (string)....... : 0.00217849s
  - big memory cache.......... : 0.00111104s
  - big memory cache (binary). : 0.0009328s