Callback from T -> U

[onmyway133]

Promise wants to hold an array of callbacks, each of which is a Callback with completion from T -> any arbitrary type U

The idea is to map from Promise<T> -> Promise<U> using then: T -> U
The problem is then acts as both subscribe and map

[onmyway133]

For now, Result<T> -> Result<U> does not hint the generic engine, so we must use as to use hint from the returned type Promise<U>

[onmyway133]

Use register block, basically, it is Result<T> -> Void, and returned promise do its notify inside this register block