Add settable promise

[art-w]

Another question mark, I'm not sure that this PR fits with domainslib general design.

I was playing with a lockfree MPMC queue and noticed that it would work well as a substitute for domainslib's Chan. The only catch is "what to do when the queue is empty and we want to pop?", because I would prefer to suspend only the current task, and not lock the current domain entirely as it could go do other stuff in the mean time. The simplest synchronization primitive I could think of is a mutable promise, such that the pop task can await it and the matching push will resolve it.

• I didn't actually replace Chan just yet, because it is currently independent from Task and either module can be used without the other -- is this intended or would it make sense to specialize Chan to maximize performances when used with tasks?

A small benchmark to demonstrate that the proposed API yields more stable performances than the alternatives:

num_domains:      1        2        3        4        5        6        7        8   
  Mpmc_pool:     57.88    63.12    52.85    61.69    51.93    52.57    64.47   394.69  <-- uses the new Task.promise
 Mpmc_retry:     60.60    71.01    83.98   110.93   202.37   155.83   192.37  1650.86  <-- uses spinlock, same impl as above
       Chan:    178.39   259.55   349.11   415.49   417.29   625.99   634.01  1221.45  <-- Domainslib.Chan

(num_domains is actually "+ 1" hence the drop at 9 domains :/)

(cc @bartoszmodelski in case you have some thoughts on this! ^^)

[art-w]

The three lines above are the fancy bit! When pop fails to obtain a value, it pushes a function onto the queue that will be called by the matching push to unlock the task :)

[kayceesrk]

Any blocking parallel data structures such as promises should be included in Saturn with concurrency libraries utilising such blocking data structures should be using domain-local await. CC @Sudha247 @polytypic.