Add ANS entropy structs

Add JxlAnsEntry and JxlAnsSymbol.

See ans_common.h. These correspond to the Entry and Symbol structures within AliasTable.

src/ImageSharp/Formats/Jxl/IO/JxlAnsEntry.cs

@@ -0,0 +1,31 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.Runtime.InteropServices;
+
+namespace SixLabors.ImageSharp.Formats.Jxl.IO;
+
+[StructLayout(LayoutKind.Sequential)]
+internal struct JxlAnsEntry
+{
+    // Although the Entry struct looks like this:
+    //     uint8_t cutoff;
+    //     uint8_t right_value;
+    //     uint16_t freq0;
+    //     uint16_t offsets1;
+    //     uint16_t freq1_xor_freq0;
+    // and clearly uses smaller types (e.g. byte or ushort),
+    // prefer using int here as otherwise we have to
+    // introduce many casts: some to assign values, others to
+    // convert from unsigned to signed kinds.
+    //
+    // This struct is 20 bytes which is more than the recommended
+    // maximum of 16 bytes, but I believe it justifies more due to
+    // reduced heap allocations that would be introduced if this
+    // struct would be turned into a class.
+    public int Entry;
+    public int RightValue;
+    public int Frequency0;
+    public int Offsets1;
+    public int Frequency1XorFrequency0;
+}

src/ImageSharp/Formats/Jxl/IO/JxlAnsSymbol.cs

@@ -0,0 +1,14 @@
+// Copyright (c) Six Labors.
+// Licensed under the Six Labors Split License.
+
+using System.Runtime.InteropServices;
+
+namespace SixLabors.ImageSharp.Formats.Jxl.IO;
+
+[StructLayout(LayoutKind.Sequential)]
+internal struct JxlAnsSymbol(int value, int offset, int frequency)
+{
+    public int Value = value;
+    public int Offset = offset;
+    public int Frequency = frequency;
+}