Add Bundle.Init (analogous to VecInit / Bundle.Lit)

I sketched out a version of this here: https://scastie.scala-lang.org/D0948BD1SgeZEqLGkVBoJg

Ideally this (and VecInit for that matter) would:
1) compose
2) utilize DataView -- (we probably cannot change VecInit but views here so that you can connect to the underlying fields would be nice)

Regardless, this sketch is useful on its own even without the more robust solution

**Type of issue**: bug report | feature request | documentation | other enhancement

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

**Development Phase**: proposal

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

Useful API for users

Start with the linked Scastie sketch and review the existing VecInit and Bundle.Lit APIs in the Chisel codebase. Determine how Bundle.Init should compose and whether DataView support is in scope; done means an agreed API addition with coverage for the intended use case.

Written by the indexing model from the issue text.