fix(core): Converts boat packets (#416)

- [X] Converts boat packets
- [X] Makes a packet container builder for packet 0xF7
- [X] Generalizes world item packet so it works for mobiles too

Notes:
- This PR doesn't address proper smooth movement for boats.

Projects/Server/Network/Packets/PacketContainerBuilder.cs

@@ -0,0 +1,115 @@
+/*************************************************************************
+ * ModernUO                                                              *
+ * Copyright (C) 2019-2021 - ModernUO Development Team                   *
+ * Email: hi@modernuo.com                                                *
+ * File: PacketContainerBuilder.cs                                       *
+ *                                                                       *
+ * This program is free software: you can redistribute it and/or modify  *
+ * it under the terms of the GNU General Public License as published by  *
+ * the Free Software Foundation, either version 3 of the License, or     *
+ * (at your option) any later version.                                   *
+ *                                                                       *
+ * You should have received a copy of the GNU General Public License     *
+ * along with this program.  If not, see <http://www.gnu.org/licenses/>. *
+ *************************************************************************/
+
+using System;
+using System.Buffers;
+using System.Buffers.Binary;
+using System.Runtime.CompilerServices;
+
+namespace Server.Network
+{
+    public ref struct PacketContainerBuilder
+    {
+        public const int MinPacketLength = 5;
+
+        private bool _finished;
+        private int _count;
+
+        private byte[]? _arrayToReturnToPool;
+        private Span<byte> _bytes;
+
+        public PacketContainerBuilder(Span<byte> initialBuffer)
+        {
+            _arrayToReturnToPool = null;
+            _finished = false;
+            _count = 0;
+
+            _bytes = initialBuffer;
+            _bytes[0] = 0xF7;         // Packet ID
+            Length = MinPacketLength; // Length + Count
+        }
+
+        public int Length { get; set; }
+
+        public int Capacity => _bytes.Length;
+
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        public ReadOnlySpan<byte> Finalize()
+        {
+            if (!_finished)
+            {
+                BinaryPrimitives.WriteUInt16BigEndian(_bytes.Slice(1, 2), (ushort)Length);
+                BinaryPrimitives.WriteUInt16BigEndian(_bytes.Slice(3, 2), (ushort)_count);
+            }
+
+            return _bytes.SliceToLength(Length);
+        }
+
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        public Span<byte> GetSpan(int bytesNeeded)
+        {
+            if (_finished)
+            {
+                throw new InvalidOperationException("Attempted to use PacketContainerBuilder after finalize");
+            }
+
+            if (Length > _bytes.Length - bytesNeeded)
+            {
+                Grow(bytesNeeded);
+            }
+
+            return _bytes.Slice(Length);
+        }
+
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        public void Advance(int bytesWritten)
+        {
+            if (_finished)
+            {
+                throw new InvalidOperationException("Attempted to use PacketContainerBuilder after finalize");
+            }
+
+            _count++;
+            Length += bytesWritten;
+        }
+
+        [MethodImpl(MethodImplOptions.NoInlining)]
+        private void Grow(int additionalCapacityBeyondPos)
+        {
+            var newLength = Math.Max(Length + additionalCapacityBeyondPos, _bytes.Length * 2);
+            byte[] poolArray = ArrayPool<byte>.Shared.Rent(newLength);
+
+            _bytes.SliceToLength(Length).CopyTo(poolArray);
+
+            byte[]? toReturn = _arrayToReturnToPool;
+            _bytes = _arrayToReturnToPool = poolArray;
+            if (toReturn != null)
+            {
+                ArrayPool<byte>.Shared.Return(toReturn);
+            }
+        }
+
+        [MethodImpl(MethodImplOptions.AggressiveInlining)]
+        public void Dispose()
+        {
+            byte[]? toReturn = _arrayToReturnToPool;
+            this = default; // for safety, to avoid using pooled array if this instance is erroneously appended to again
+            if (toReturn != null)
+            {
+                ArrayPool<byte>.Shared.Return(toReturn);
+            }
+        }
+    }
+}