Consider implementing higher-order 'Amb' overload

[bruno-f-cruz]

This may be ambiguous with the single input overload. However, the single input is also not consistent with the Concat operator. (#1000)

This addition would allow multiple sequences to be subscribed as they are produced and, once one of them produces a value, propagate said sequence and close all others.

Since this overload doesn't seem to exist in rx, we should probably wait to check what comes out of dotnet/reactive#2165