std::rand: move normal and exponential to their own file.
std::rand: implement the log-normal distribution.
std::rand: implement the chi-squared distribution.
std::rand: implement the F distribution.
std::rand: implement the student t distribution.
auto merge of #10859 : huonw/rust/helper-dists, r=cmr

This moves `std::rand::distribitions::{Normal, StandardNormal}` to `...::distributions::normal`, reexporting `Normal` from `distributions` (and similarly for `Exp` and Exp1`), and adds:
- Log-normal
- Chi-squared
- F
- Student T

all of which are implemented in C++11's random library. Tests in https://github.com/huonw/random-tests/commit/0424b8aded5e608ae386c1f917934a726d9cac6a. Note that these are approximately half documentation & half implementation (of which a significant portion is boilerplate `}`'s and so on).