implicit typecast from shorter OneOfs

[Pxtl]

allow implicit typecast of shorter OneOfs to longer OneOfs whose first classes match the short one in full.

This allows the following:

  OneOf<A, B, C> GetOneOfABC()
    => DoSomething();
    
  OneOf<A, B, C, D> GetOneOfABCD()
    => (SomeDPredicate()) 
    ? GetD()
    : GetOneOfABC(); //this is the cool part

where GetD() returns some D, and DoSomething() returns a OneOf<A, B, C>

[emperador-ming]

This would be awesome!