Chisel plugin bundle elements handler (#2306)

Adds generation of `Bundle.elements` method to the chores done by the compiler plugin
For each `Bundle` find the relevant visible Chisel field members and construct a
hard-coded list of the elements and their names implemented as `_elementsImpl`

For more details: See plugins/README.md

- Should be no change in API
- Handles inheritance and mixins
- Handles Seq[Data]
- Tests in BundleElementSpec

Co-authored-by: chick <chick.markley@sifive.com>
Co-authored-by: Jack Koenig <koenig@sifive.com>