no_std support #123
Description
I think it's worth to add support for no_std. For example I am planning to implement Fortuna algorithm with no_std support (so it could be used on micro controllers) and it would be good to implement Rng trait so code implemented for it will be reused.
It can be done using std (or use-std, it seems there is currently no agreement on this) feature which will be enabled by default.