[ChiselSim] ChiselEnum does not provide peek/poke methods and user-friendly API

While rewriting some tests from chiseltest to ChiselSim, I've noticed ChiselEnum is not handled by the Peek/Poke API.

In a test with `c.io.inst.peek() should be(inst)` where `inst` is a `ChiselEnum.Type`, I get: `value peek is not a member of chiselv.Instruction.Type`.

The closest thing I could get to work is

```scala
c.io.inst.peekValue().asBigInt should be(inst.litValue)
```

Which is very convoluted and not user-friendly.