Implement an efficient groupBy in a later release

[rotexdegba]

See

• https://github.com/jakezatecky/array_group_by ,
• https://gist.github.com/mcaskill/baaee44487653e1afc0d ,
• https://gist.github.com/rotexdegba/99b0f120ddf4fe0a93b2a38dbff84a66 ,
• http://wesmckinney.com/blog/mastering-high-performance-data-algorithms-i-group-by/

for inspiration. Also look at the Laravel collections implementation.