std::rand: implement the Gamma distribution

Implements the [Gamma distribution](https://en.wikipedia.org/wiki/Gamma_distribution), using the algorithm described by Marsaglia & Tsang 2000[1].

Also, moves `std::rand::distributions` to a subfolder, and performs a minor clean-up of the benchmarking (makes the number of iterations shared by the whole `std::rand` subtree).

[1]: George Marsaglia and Wai Wan Tsang. 2000. "A Simple Method for Generating Gamma Variables" *ACM Trans. Math. Softw.* 26, 3 (September 2000), 363-372. DOI:[10.1145/358407.358414](http://doi.acm.org/10.1145/358407.358414).