crucible-mir: Remove vestigial ?mirCS and ?mirLib implicit params, as well as Monoid instances for Collection/CollectionState/RustModule

[RyanGlScott]

In #1253, we propose to make crucible-mir respect the mir-json schema version. As part of implementing this, it would be convenient to store the version number in the parsed Collection, as this could be useful to report to the user in various situations. However, there is an obstacle that stands in our way of doing so: Collection has a Monoid instance, and it's very unclear how to implement mempty and mappend for version numbers. We certainly don't want to conjure up a version number out of thin air in mempty, and it's unclear how we would combine two different version numbers.

Why does Collection have a Monoid instance anyway? It ultimately dates back to an old approach that crux-mir used for translating Collections in commit 12df289. In that approach, one would call translateMIR multiple times and combine the results using mappend. However, that approach was removed in commit b0708a0. After that commit, the only time we ever use the Monoid instance for Collection is to call mappend (mempty :: Collection) x. This could just as well be simplified to x, thereby avoiding the need for the Monoid instance in the first place.

Similarly, we could remove:

• The Monoid instances for CollectionState and RustModule (which depend on the Monoid Collection instance, but are similarly unused)
• The ?mirCS and ?mirLib implicit parameters (which are only ever instantiated with mempty).