small optimizations

[DiamonDinoia]

This PR:

1. uses cmake to fetch libdivide to avoid polluting git history
2. Uses std functionality for clarity and speed
3. increases performance to 461MB/s and 672 MB/s

[magland]

Thanks @DiamonDinoia

If you don't mind, I'd like to consider each of these changes separately. In particular, it would be great to pinpoint exactly which change leads to the performance improvement.

If you have the time, separate small PRs would be very helpful.

[DiamonDinoia]

The more logical way I can think of splitting this is:
PR1. update the dependencies
PR2. use the stdlib more

Is this granular enough?

[magland]

It looks to me like there are more changes to simple_ans.hpp than just moving toward using stdlib more. It would be nice to consider the changes individually to find out which gives the performance boost.