Add initial scaffold for Root Hash calculation

src/Paprika.Tests/Merkle/RootHashTests.cs

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+using NUnit.Framework;
+using Paprika.Chain;
+using Paprika.Crypto;
+using Paprika.Merkle;
+using Paprika.Store;
+
+namespace Paprika.Tests.Merkle;
+
+public class RootHashTests
+{
+    private const int SmallDb = 256 * Page.PageSize;
+    private readonly Keccak _blockKeccak = Keccak.Compute("block"u8);
+
+    [Test]
+    public async Task Empty_database()
+    {
+        using var db = PagedDb.NativeMemoryDb(SmallDb);
+        await using var blockchain = new Blockchain(db, preCommit: new ComputeMerkleBehavior());
+
+        using var block = blockchain.StartNew(Keccak.Zero, _blockKeccak, 1);
+
+        block.Commit();
+
+        var rootHash = block.GetMerkleRootHash();
+        var expectedRootHash =
+            new Keccak(Convert.FromHexString("56E81F171BCC55A6FF8345E692C0F86E5B48E01B996CADC001622FB5E363B421"));
+
+        Assert.That(rootHash, Is.EqualTo(expectedRootHash));
+    }
+
+    [Test]
+    [Ignore("Not working yet")]
+    public async Task Single_account()
+    {
+        using var db = PagedDb.NativeMemoryDb(SmallDb);
+        await using var blockchain = new Blockchain(db, preCommit: new ComputeMerkleBehavior());
+
+        var key = Values.Key0;
+        var account = new Account(Values.Balance0, Values.Nonce0);
+
+        using var block = blockchain.StartNew(Keccak.Zero, _blockKeccak, 1);
+
+        block.SetAccount(key, account);
+        block.Commit();
+
+        var rootHash = block.GetMerkleRootHash();
+        var expectedRootHash =
+            new Keccak(Convert.FromHexString("E2533A0A0C4F1DDB72FEB7BFAAD12A83853447DEAAB6F28FA5C443DD2D37C3FB"));
+
+        Assert.That(rootHash, Is.EqualTo(expectedRootHash));
+    }
+}

src/Paprika/Merkle/ComputeMerkleBehavior.cs

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+using Paprika.Chain;
+
+namespace Paprika.Merkle;
+
+public class ComputeMerkleBehavior : IPreCommitBehavior
+{
+    public void BeforeCommit(ICommit commit)
+    {
+        // Foreach key in the commit:
+        //      > Set each intermediate Merkle node as 'Dirty'
+        // Modify any intermediate Merkle node if there were any inserts (7 cases)
+        // Recompute the Keccak of all Merkle nodes
+        // The root Merkle node should exist on the Empty Path (''), and it's Keccak is the Merkle Root Hash
+    }
+}

src/Paprika/Merkle/WorldStateExtensions.cs

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+using Paprika.Chain;
+using Paprika.Crypto;
+
+namespace Paprika.Merkle;
+
+public static class WorldStateExtensions
+{
+    public static Keccak GetMerkleRootHash(this IWorldState state)
+    {
+        return Keccak.EmptyTreeHash;
+    }
+}