Invalid return for signed bytes in GetNumberSize

[jtsylve]

https://github.com/polenter/SharpSerializer/blob/55782864a46016c663779688b400026bcb354d4a/SharpSerializer/Core/Binary/Elements.cs#L226

This function returns an invalid number of bytes for signed 8-bit integers. I believe the check should be referencing SByte.MinValue and SByte.MaxValue rather than byte.