DeserializeAsync fully consumes FileStreams

[bartimaeusnek]

When using MemoryPackSerializer.DeserializeAsync with a FileStream, the expectation is that after deserializing an object from the stream, the stream's position should move forward only by the number of bytes required to deserialize that specific object. However, after the first call to DeserializeAsync, the stream is fully consumed, making it impossible to deserialize subsequent objects from the same stream without resetting the position.

Other types of Streams might also be affected, i havent tested that tho.

Example Code:

    public async Task MemoryPackBug()
    {
        await using var fs = new FileStream(Path.GetTempFileName(), FileMode.OpenOrCreate, FileAccess.ReadWrite);
        await MemoryPackSerializer.SerializeAsync(fs, "First String");
        var positionOne = fs.Position;
        await MemoryPackSerializer.SerializeAsync(fs, "Second String");
        var positionTwo = fs.Position;
        
        fs.Seek(0, SeekOrigin.Begin);
        
        _ = await MemoryPackSerializer.DeserializeAsync<string>(fs);
        if(fs.Position != positionOne) {
            throw new Exception();
        }
        _ = await MemoryPackSerializer.DeserializeAsync<string>(fs);
        if(fs.Position != positionTwo) {
            throw new Exception();
        }
    }

With the current version of MemoryPack (1.21.3) on .NET8 this throws at the if(fs.Position != positionOne)