Implement ToArray, ToDictionary, ToList, and ToLookup

[jasonboukheir]

https://docs.microsoft.com/en-us/dotnet/api/system.linq.enumerable.toarray?view=netframework-4.7.1 https://docs.microsoft.com/en-us/dotnet/api/system.linq.enumerable.todictionary?view=netframework-4.7.1 https://docs.microsoft.com/en-us/dotnet/api/system.linq.enumerable.tolist?view=netframework-4.7.1 https://docs.microsoft.com/en-us/dotnet/api/system.linq.enumerable.tolookup?view=netframework-4.7.1

[jasonboukheir]

Since these classes aren't usable inside Burst, I think it's better not to implement these for now.