Add valgrind annotations

This appears to affect the binary (how else could it possibly work??),
but the microbenchmarks don't show a difference

ConflictSet.cpp

@@ -36,6 +36,11 @@ limitations under the License.
 #include <arm_neon.h>
 #endif
 
+#if __has_include(<valgrind/memcheck.h>)
+#define MEMCHECK
+#include <valgrind/memcheck.h>
+#endif
+
 // ==================== BEGIN IMPLEMENTATION ====================
 
 struct Entry {
@@ -53,10 +58,16 @@ struct BoundedFreeListAllocator {
       return new (safe_malloc(sizeof(T))) T;
     }
     assert(freeList != nullptr);
-    void *result = freeList;
+    void *buffer = freeList;
+#ifdef MEMCHECK
+    VALGRIND_MAKE_MEM_DEFINED(freeList, sizeof(freeList));
+#endif
     memcpy(&freeList, freeList, sizeof(freeList));
     --freeListSize;
-    return new (result) T;
+#ifdef MEMCHECK
+    VALGRIND_MAKE_MEM_UNDEFINED(buffer, sizeof(T));
+#endif
+    return new (buffer) T;
   }
 
   void release(T *p) {
@@ -67,10 +78,16 @@ struct BoundedFreeListAllocator {
     memcpy((void *)p, &freeList, sizeof(freeList));
     freeList = p;
     ++freeListSize;
+#ifdef MEMCHECK
+    VALGRIND_MAKE_MEM_NOACCESS(p, sizeof(T));
+#endif
   }
 
   ~BoundedFreeListAllocator() {
     for (void *iter = freeList; iter != nullptr;) {
+#ifdef MEMCHECK
+      VALGRIND_MAKE_MEM_DEFINED(iter, sizeof(iter));
+#endif
       auto *tmp = iter;
       memcpy(&iter, iter, sizeof(void *));
       free(tmp);