Test with huge number of actors

[ddrcode]

Prepare a test case with a huge number of actors (thousands) - i.e. some simulation scenario.
Use it to test:

• stability
• performance
• test harness ergonomics

Steps

• Define application scenario
• Implement the app inside tests (or examples, or even separate crate if needed?)
• Implement tests with Maiko's test harness - focus on ergonomics
• Implement bench for the example - focus on event propagation time
• Learn from the experience. Create tickets for all observed API, performance and usability issues.

[ddrcode]

I've generated test-particle-swarm project exactly for that reason - testing scenario of multiple actors. I didn't want to squeeze it into the main repo. Apply any changes there