Support literals and DontCare in DataView targets

[jackkoenig]

The best example of why this is useful is the test showing some possible uses with Valid:

object ValidExtensions {
  implicit def view[T <: Data] = DataView[T, Valid[T]](
    x => Valid(x.cloneType), // Valid will strip direction with `Output(...)` anyway
    _ -> _.bits,
    (_, v) => true.B -> v.valid
  )
}

This enables you to box any T as a Valid[T] where the valid bit is the literal true:

import ValidExtensions._
class MyModule extends Module {
  val in = IO(Input(UInt(8.W)))
  val out= IO(Output(Valid(UInt(8.W))))
  out := in.viewAs[Valid[UInt]]
}

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[seldridge]

Nice feature improvement. The valid view is pretty slick.

[seldridge]

Should this be a public utility?

[jackkoenig]

Probably, we need to figure out a general coding style for such methods. I think it should probably be a factory method on object Valid rather than making people call .viewAs[Valid[CorrectType]]. The problem is that the natural answer would be Valid.apply, but that is obviously taken by the factory for making types rather than values. It's sort of the "VecInit" pattern (but obviously that name is not the best), and if we can come up with a decent "standard", I think this would be a great example of it.