Add benchmark for #flat_map

Calculating -------------------------------------
.map.flatten with nested arrays
                          4718 i/100ms
.flat_map with nested arrays
                          6048 i/100ms
.map.flatten with no nested arrays
                          9804 i/100ms
.flat_map with no nested arrays
                          9302 i/100ms
-------------------------------------------------
.map.flatten with nested arrays
                        48118.3 (±4.8%) i/s -     240618 in   5.011942s
.flat_map with nested arrays
                        63838.6 (±5.1%) i/s -     320544 in   5.034864s
.map.flatten with no nested arrays
                       104879.3 (±4.4%) i/s -     529416 in   5.057802s
.flat_map with no nested arrays
                        99935.3 (±6.6%) i/s -     502308 in   5.049506s