Implement vararg methods

[MichalStrehovsky]

Low priority. Not in the .NET Core profile.

Depends on #367, which is another low priority workitem.

[MichalStrehovsky]

Note this also includes work on the reflection side (making sure this is properly represented in the metadata format and attempts to reflection-invoke these methods throws the right exception type).

[jkotas]

This is not needed for .NET Core. It would be only needed if we ever wanted to target full .NET Framework surface using the CoreRT compiler.

[MichalStrehovsky]

As per dotnet/standard#20 this will be part of the .NET Standard. Exact prioritization TBD, but reopening.