use Vec to handle out-of-order items

[aawsome]

The # of out-of-order items is IMO bounded by the # of threads.
In any case it is relatively small and I don't see why a HashMap should perform any better than a simple Vec (especially given that the HashMap uses Vec-like buckets which are simple sequentially searched - I could imagine that we typically have a HashMap with just 1 bucket...)

[aawsome]

Here are some benchmarks (master vs. vec-out-of-order):

fibonacci(10)/parallel_map-t1/100                                                                                                      
                 change:                                                                                                               
                        time:   [-3.9826% -1.0928% +1.7553%] (p = 0.47 > 0.05)                                                         
                        thrpt:  [-1.7250% +1.1048% +4.1478%]                                                                           
                        No change in performance detected.
fibonacci(10)/parallel_map-t2/100                                                                             
                 change:
                        time:   [-1.4672% -0.1405% +1.2323%] (p = 0.84 > 0.05)
                        thrpt:  [-1.2173% +0.1407% +1.4890%]
                        No change in performance detected.
fibonacci(10)/parallel_map-t4/100                                                                             
                 change:
                        time:   [-6.5400% -1.9073% +2.8612%] (p = 0.44 > 0.05)
                        thrpt:  [-2.7816% +1.9444% +6.9977%]
                        No change in performance detected.
fibonacci(10)/parallel_map-t1/1000                                                                             
                 change:
                        time:   [-4.9744% -1.0625% +2.9276%] (p = 0.59 > 0.05)
                        thrpt:  [-2.8443% +1.0739% +5.2348%]
                        No change in performance detected.
fibonacci(10)/parallel_map-t2/1000                                                                             
                 change:
                        time:   [-8.3858% -7.4055% -6.4442%] (p = 0.00 < 0.05)
                        thrpt:  [+6.8881% +7.9977% +9.1533%]
                        Performance has improved.
fibonacci(10)/parallel_map-t4/1000                                                                             
                 change:
                        time:   [-13.793% -9.9091% -6.0077%] (p = 0.00 < 0.05)
                        thrpt:  [+6.3917% +10.999% +15.999%]
                        Performance has improved.
fibonacci(10)/parallel_map-t1/10000                                                                              
                 change:
                        time:   [-11.191% -6.3192% -1.2853%] (p = 0.02 < 0.05)
                        thrpt:  [+1.3021% +6.7454% +12.602%]
                        Performance has improved.
fibonacci(10)/parallel_map-t2/10000                                                                              
                 change:
                        time:   [-13.405% -12.363% -11.324%] (p = 0.00 < 0.05)
                        thrpt:  [+12.771% +14.107% +15.480%]
                        Performance has improved.
fibonacci(10)/parallel_map-t4/10000                                                                              
                 change:
                        time:   [-18.851% -16.964% -15.191%] (p = 0.00 < 0.05)
                        thrpt:  [+17.912% +20.429% +23.230%]
                        Performance has improved.

[dpc]

@aawsome I think you're right, good thinking. Thanks!

[dpc]

@aawsome If you don't mind, if you could update CHANGELOG and add a comment right next to that Vec about why it's a vec, that would be awesome. Otherwise I'll try to remember to do it later. :)