Improve data-structures.md #16
Description
I got some good feedback on reddit https://www.reddit.com/r/haskell/comments/1pgwhru/short_overview_of_common_data_structures_in , mainly:
- Real benefits (memory and speed) come from Vector and Array when using their Storable/Unboxed version, because otherwise they are using a bunch of pointers that slow things down (I guess still one big chunk of memory is behind those pointers, or not even that maybe?).
- I should better explain why Map is relatively fast, what does that mean.