…ute mapping (JAVA-274)

Commit improves the yet-to-be-released SentryNavEffect by replacing separate route name and argument extractors with a single BackStackEntryMapper. Doing so lets us spare users from having to create three giant `when` statements mapping all nav entries in their entire app (two for us and one for Nav3's entryProvider). After this commit, users only have to create two. (Future work will allow them to create just one via a forthcoming Sentry entryProvider wrapper.)

Commit also uses the term "back stack entry" rather than "back stack route" throughout to avoid developer confusion, given Google's use of "route" to mean (essentially) a navigation destination. By contrast, we need a term that refers solely to an element in the host app's back stack. (In general, Nav3 is careful to distinguish between nav destinations and back stack entries, as a destination may be composed from multiple entries in a back stack.)

Commit removes our Nav3 TypeCheckHint for breadcrumbs, which was cargo-culted from Nav2. We don't need it because - unlike Nav2 - developers can scrub Nav3 data before it's recorded. Our public KDocs instruct them to do so for *all* Sentry Nav3 data, not just breadcrumbs.

Supporting a mechanism for after-the-fact access to just breadcrumb data (and not other Nav3 data) would be both asymmetrical and superfluous.