Extends CompletionStage with MultiCompletionStage

[linzee1]

It is generally reccommand to use interface rather than specific class. Many users use CompletionStage despite of the underlying CompletableFuture.
There are a bunch of new methods provided by CFFU, such as

1. Cffu<List> thenMApplyFastFailAsync(Function<? super T, ? extends U>... fns)
2. Cffu<Tuple2<U1, U2>> thenTupleMApplyFastFailAsync(Function<? super T, ? extends U1> fn1, Function<? super T, ? extends U2> fn2),

which are basicly extentions of stage operations.
Maybe it is better to use a new subinterface like MultiCompletionStage. MultiCompletionStage should contain the methods below

Possible problem: this change could make the project much more complex.