Some recommendations on stop

1. Add `stop(code: UInt): Unit`. I am not sure whether it is possible. It could be more useful if `stop` can pass the internal circuit state outside.
2. Add `finish()`, finish has different meanings other than stop. A user-intended finish is very useful for simulation control.

https://github.com/freechipsproject/chisel3/blob/473a13877c60ba9fb13de47542a8397412c2b967/core/src/main/scala/chisel3/Assert.scala#L80-L92