Allow stack to grow in gc

The previous bound was valid in each logical version of the map, but not
for the physical map's structure.

VersionedMap.cpp

@@ -285,9 +285,11 @@ struct MemManager {
   void gc(const uint32_t *roots, int numRoots, int64_t oldestVersion) {
     // Calculate reachable set
     BitSet reachable{next};
-    // Each node has at most 3 children and nodes along the search path aren't
-    // in the stack, so we need 2 * kPathLengthUpperBound
-    uint32_t stack[2 * kPathLengthUpperBound];
+    Arena arena;
+    constexpr int kInitialStackCapacity = 128;
+    int64_t stackCapacity = kInitialStackCapacity;
+    uint32_t stackStack[kInitialStackCapacity];
+    uint32_t *stack = stackStack;
     int stackIndex = 0;
     auto tryPush = [&]([[maybe_unused]] uint32_t parent, uint32_t child) {
       if (!reachable.set(child)) {
@@ -296,7 +298,12 @@ struct MemManager {
           printf("  GC: reach: %u (parent %u)\n", child, parent);
         }
 #endif
-        assert(stackIndex < int(sizeof(stack) / sizeof(stack[0])));
+        if (stackIndex == stackCapacity) [[unlikely]] {
+          auto *old = stack;
+          stackCapacity *= 2;
+          stack = new (arena) uint32_t[stackCapacity];
+          memcpy(stack, old, stackIndex * sizeof(stack[0]));
+        }
         stack[stackIndex++] = child;
       }
     };