Require non-adjacent mutations

This should make it so that mutations can only be adjacent if they're at
different versions

include/VersionedMap.h

@@ -66,7 +66,7 @@ struct VersionedMap {
     MutationType type;
   };
 
-  /** Mutations must be sorted and non-overlapping. `version`
+  /** Mutations must be sorted, non-overlapping, and non-adjacent. `version`
    * must be strictly increasing. Postcondition: `getVersion()` == `version` */
   void addMutations(const Mutation *mutations, int numMutations,
                     int64_t version);
@@ -86,14 +86,15 @@ struct VersionedMap {
   /** The version of the most recent call to `setOldestVersion`. */
   int64_t getOldestVersion() const;
 
-  /** Iterates through a partially canonicalized[1] view of all the mutations
+  /** Iterates through a canonicalized[1] view of all the mutations
    * from `oldestVersion` to the iterator's version. There may be mutations from
    * versions < `oldestVersion`, but they won't affect the result. It's
    * thread-safe to operate on an iterator concurrently with any method of
    * `VersionedMap`, as long as it's not invalidated by `setOldestVersion`.
    * @warning must not outlive its `VersionedMap`.
    *
-   * [1]: Mutations are sorted and non-overlapping, but may be adjacent.
+   * [1]: Mutations at different versions may be adjacent. This is necessary for
+   * precisely tracking at what version the mutations take effect.
    */
   struct Iterator {