TypeResolutionTable

As described in #91.
I just found out that there's [`ImmutableHashSet<T>`](http://msdn.microsoft.com/en-us/library/dn467171.aspx), only available via [Microsoft.Bcl.Immutable](https://www.nuget.org/packages/Microsoft.Bcl.Immutable) NuGet package. However, I haven't used it before and I'm not aware of its performance characteristics...