Do not enforce Branch size

- Keccaks will be optional, so Size wont be known AOT

src/Paprika.Tests/Merkle/NodeTests.cs

@@ -10,7 +10,7 @@ public class NodeTests
 {
     [Test]
     [TestCase(typeof(Node.Header), 1)]
-    [TestCase(typeof(Node.Branch), 35)]
+    [TestCase(typeof(Node.Branch), 64)]
     [TestCase(typeof(Node.Leaf), 64)]
     [TestCase(typeof(Node.Extension), 32)]
     public void Struct_size(Type type, int expectedSize)
@@ -101,7 +101,7 @@ public void Branch_read_write(ushort nibbleBitSet, Keccak keccak)
     {
         var branch = new Node.Branch(nibbleBitSet, keccak);
 
-        Span<byte> encoded = stackalloc byte[Node.Branch.Size];
+        Span<byte> encoded = stackalloc byte[branch.MaxByteLength];
         _ = branch.WriteTo(encoded);
         _ = Node.Branch.ReadFrom(encoded, out var decoded);
 
@@ -214,7 +214,7 @@ public void Node_read_branch()
         var keccak = Values.Key0;
 
         var branch = new Node.Branch(nibbleBitSet, keccak);
-        Span<byte> buffer = stackalloc byte[Node.Branch.Size];
+        Span<byte> buffer = stackalloc byte[branch.MaxByteLength];
         _ = branch.WriteTo(buffer);
         _ = Node.ReadFrom(buffer, out var nodeType, out _, out _, out var actual);
 

src/Paprika/Merkle/MerkleNode.cs

@@ -197,19 +197,13 @@ public override string ToString() =>
             $"}}";
     }
 
-    [StructLayout(LayoutKind.Explicit, Pack = 1, Size = Size)]
     public readonly ref struct Branch
     {
-        public const int Size = 35;
+        public int MaxByteLength => 35;
         private const int NibbleBitSetSize = sizeof(ushort);
 
-        [FieldOffset(0)]
         public readonly Header Header;
-
-        [FieldOffset(1)]
         public readonly ushort NibbleBitSet;
-
-        [FieldOffset(3)]
         public readonly Keccak Keccak;
 
         // TODO: What interface do we want to expose for nibbles?