Use vectors instead of lazy seqs
Use reset! instead of set!
Fix the test of sending to self

There's no need to start the actor again in its body. It will be restarted
automatically.
Remove redundant do in the body of catch
All test actors die gracefully after tests
Tests: replace swap! with reset! where appropriate
Stylistic changes in tests (doall · map → doseq)
Extract the common let form in the core/new macro
Remove core/stop

Introducing it was a bad idea. Its meaning was unclear and there were no
guarantees that it would kill an actor which was waiting for a new message.

Hypothetically, actors could try to acquire their semaphores with a
timeout, check whether core/stop was called on them, and react
appropriately. Still, it wouldn't be a good idea because actors whose
callbacks take a longer while wouldn't be killed immediately. Such
abstraction is rather leaky.

What seems to make more sense is killing actors by defining a callback
which calls the die function, as in:

    (himilsbach.core/new
       msg    (do-something msg)
       [:die] (die))
kill-order should not be touched
Minor, unimportant changes in the benchmark
Add doc strings to the public interface
Update the year in the copyrights line
Add an example to the README.md
Simplify the example in the README.md a bit