Added sources for an open source implementation of System.Collections.Immutable

Original source: https://github.com/mono/ImmutableCollections
Commit: 4f149018cfba40c89c5b5cd62ccb8efc7c39c759
ImmutableDictionary's public interface matches that of the Microsoft.Bcl.Immutable NuGet package version 1.0.30
Non-Windows builds refer to the project System.Collections.Immutable instead of the NuGet package