New callback dispatcher for .NET Standard 2.1

[Akarinnnnn]

Breaking changes included

1. Enabled unsafe code to use Pointers for heap Allocation Free
2. Changed event handler delegate's signature, use ref T instead of T to avoid copy the event param.
3. Throw System.ArgumentException instead of System.Exception when user passed an invalid handler while creating Callresult<T> and Callback<T>

[Akarinnnnn]

Examples for new handler

// For Callresult<T>
void OnCallresult(ref TypeName param, bool iofail)
{
    // do your operations
}

// For Callback<T>
void OnCallback(ref TypeName param)
{
    // do your operations
}

[rlabrecque]

I don't think we can do 1. as enabling unsafe on Unity is a pain. 2. & 3. seem good but we should probably wait until other breaking changes around here maybe: https://github.com/rlabrecque/Steamworks.NET/issues/389

[Akarinnnnn]

I don't think we can do 1. as enabling unsafe on Unity is a pain. 2. & 3. seem good but we should probably wait until other breaking changes around here maybe: #389

I will add some #ifdef later to enable these pointer tricks just on .NET Core platforms. And we can wait for #389 for enough time to evaluate how this breaking changes impacts.

[Akarinnnnn]

The new dispatcher was moved to a new file now.