Tweak Bundle._elementsImpl (#2390) (#2392)

* Change type of Bundle._elementsImpl to Iterable

It was previously SeqMap (ListMap on Scala 2.12). This change gives us
more freedom to optimize the implementation without breaking binary
compatibility. It is scala.collection.Iterable because it is perfectly
fine to return mutable collections (like Arrays) since the only use is
to Iterate on them.

* Disallow users implementing Bundle._elementsImpl

Currently, it would result in a runtime linkage error. This turns it
into a compile-time error.

Co-authored-by: mergify[bot] <37929162+mergify[bot]@users.noreply.github.com>
(cherry picked from commit 1b05a14a

)

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