TopWiring API

**Type of issue**: feature request

**Impact**: API addition (no impact on existing code)

**Development Phase**: request

**What is the use case for changing the behavior?**

`TopWiringAnnotation` has been around for a while, but there is not Chisel-level API. It would be good to think about adding this in. The API for this can likely be pretty simple. Something like:

```scala
class Foo extends RawModule {
val bar = RegInit(0.U(1.W))
BoringUtils.shaft(bar)
}
```

Bikeshedding the method name:
- `topWire`
- `boreUp`
- `boreTop`
- `chimney`
- `shaft`
- `boreShaft`
- `mineShaft`
- `debug`
- `via`