Add Promise.cache

[kevinresol]

Promise.cache returns a function which when called:

• create a promise if it is the first call; OR
• reuse the previous promise if not yet expired; OR
• create a new promise if the previous one is expired

Sample use:

var example = Promise.cache(fetchExampleDotCom, triggerNoiseAfterDelayMs(1000));
example(); // will create a new promise that fetches example.com
example(); // will return the same promise (i.e. won't fetch again)

// after 1000ms
example(); // will fetch example.com again

[back2dos]

Hmm, I wonder if it's might be better to do it this way:

function cache<T>(gen:Void->Promise<Pair<T, Future<Noise>>):Void->Promise<T> { ... }

Mostly because otherwise if expiration should be somehow tied to data, you have to share state between the functions somehow. But I'm just thinking out loud.

[kevinresol]

Updated, also allowed sync expire

[kevinresol]

Well Java choked on this:

src/tink/core/_Promise/Promise_Impl_.java:309: error: generic array creation
        tink.core._Future.FutureObject<tink.core.Outcome>[] p = ( (tink.core._Future.FutureObject<tink.core.Outcome>[]) (new tink.core._Future.FutureObject<tink.core.Outcome>[] {null}) );
                                                                                                                         ^

[back2dos]

Gotta love hxjava \o/