Fix bundle elements performance regression (#2396) (#2398)

* Only call _elementsImpl once in Bundle.elements

* Distinguish compiler plugin and reflective _elementsImpl

Bundle.elements now will only do post-processing if the user is using
plugin-generated _elementsImpl. This improves performance for the case
where the user does not opt-in to using the plugin to generate
_elementsImpl.

(cherry picked from commit 5fead89e

)

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