restructure code

[koeppl]

It would be nice to have a wrapper header file for each hash table we add, and a list where the user can choose which hash tables to include for the benchmarks.
Regrading the benchmarks, I would like to add k-mer counting from a file (for instance using a sliding window to also use the erase function).
Also, a Markov chain source should be interesting, instead of using just a uniform random source.