Adds a FLUSH io function

[enricozb]

No description provided.

[HigherOrderBot]

Perf run for 49a13b8:

compiled
========

file            runtime         main            (local)       
==============================================================
sort_bitonic    c                        5.13s           5.35s
                cuda                     0.23s           0.24s
--------------------------------------------------------------
sum_rec         c                        1.43s           1.43s
                cuda                     0.14s           0.15s
--------------------------------------------------------------
sum_tree        c                        0.15s           0.12s
                cuda                     0.10s           0.11s
--------------------------------------------------------------
tuples          c                        4.03s           3.77s
                cuda                   timeout         timeout
--------------------------------------------------------------

interpreted
===========

file            runtime         main            (local)       
==============================================================
sort_bitonic    c                        3.47s           4.70s
                cuda                     0.23s           0.23s
                rust                   timeout         timeout
--------------------------------------------------------------
sum_rec         c                        1.68s           1.68s
                cuda                     0.14s           0.14s
                rust                    13.64s          13.54s
--------------------------------------------------------------
sum_tree        c                        0.18s           0.18s
                cuda                     0.09s           0.08s
                rust                     0.87s           0.86s
--------------------------------------------------------------
tuples          c                        2.56s           3.57s
                cuda                   timeout         timeout
                rust                     3.65s           3.63s
--------------------------------------------------------------