Implement standard debuggers.

These debug actions are added to all players by default.

cardsim/sliceutil.go

@@ -145,3 +145,21 @@ func Strip[T any](slice []T, removeWhen func(idx int, t T) bool) []T {
 	}
 	return slice[:to]
 }
+
+// EnsureCapacity checks if `cap(slice)` is at least req. If so, it returns
+// slice unchanged. Otherwise, it copies `slice` to a new slice that is at least
+// capacity `req` (but may be larger) and returns the copy.
+//
+// It is reasonably efficient to use EnsureCapacity consecutively without
+// regard for the final overall capacity that a specific slice will need to be.
+func EnsureCapacity[T any](slice []T, req int) []T {
+	if cap(slice) >= req {
+		return slice
+	}
+	if req < 2*cap(slice) {
+		req = 2 * cap(slice)
+	}
+	ret := make([]T, len(slice), req)
+	copy(ret, slice)
+	return ret
+}