Change max key len to 8, update corpus

Now that we don't have a fixed buffer reserved for partial key bytes,
there's nothing (obvious) that makes testing short versus long keys much
different. maybeDecreaseCapacity is an exception, and we'll write some
tests covering that manually.

Internal.h

@@ -531,7 +531,7 @@ template <class ConflictSetImpl> struct TestDriver {
   ConflictSetImpl cs{oldestVersion};
   ReferenceImpl refImpl{oldestVersion};
 
-  constexpr static auto kMaxKeyLen = 128;
+  constexpr static auto kMaxKeyLen = 8;
 
   bool ok = true;