nqp::sleep, nqp::exit, and some cleanup of nqp::sha1

Although I've tested nqp::exit and nqp::sleep by hand, no tests of them are included.  I always get an error similar to:
````Exception in thread "main" java.lang.NoSuchMethodError: org.perl6.nqp.runtime.Ops.exit(J)Ljava/lang/String;````

That is, it's looking for exit or sleep to return a String, but I can't figure out why.