Mergesort using goroutines

[CorentinGS]

Description
It would be interesting to add a mergesort algorithm that uses goroutines. It's a good way to show how routines can be used to improve classical algorithm.

For enhancement:

• Add a ParallelMergesort function that use goroutines
• Benchmark & test it and compare how it performs against classical mergesort

I can do a pr in the weekend if you are ok with that suggestion.

[raklaptudirm]

Don't open issues for missing implementations. Just create a pull request and we will review it.