Create Future (or Task) monad

[myugen]

A Future represents a value which may become available at some point (asynchronous action). This value is usually the result of some computation.

1. If the computation has not yet completed, we say that the Future is not completed.
2. If the computation has completed with a value or with an exception, we say that the Future is completed.

Completion can take one of two forms:

1. When a Future is completed with a value, we say that the future was successfully completed with that value.
2. When a Future is completed with an exception thrown by the computation, we say that the Future was failed with that exception.

For value evaluation, this monad should provide a callback function to call once the Future is completed.