std::rand: Move distributions to a subfolder.
std::rand: Implement the Gamma distribution.
std::rand: share the benchmark counter among the whole module tree.
auto merge of #10223 : huonw/rust/gamma, r=cmr

Implements the [Gamma distribution](https://en.wikipedia.org/wiki/Gamma_distribution), using the algorithm described by Marsaglia & Tsang 2000[1]. I added tests checking that the mean and variance of this implementation is as expected for a range of values of the parameters in https://github.com/huonw/random-tests/commit/5d87c00a0fb69c8fa173593714cef76ddfddb651 (they pass locally, but obviously won't even build on Travis until this is merged).

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).
Stop propagating link arguments across crates

This is a fairly brittle modle that doesn't scale well across many crates. It's
unlikely that all of the downstream crates will have all of the original native
dependencies of all the upstream crates. In the case that FFI functions are
reachable, then it should be the responsibility of the downstream crate to link
against the correct library, or the upstream crate should prevent the functions
from being reachable.
Statically link sundown to librustdoc

Closes #10103
Statically link libuv to librustuv

Similarly to the previous commit, libuv is only used by this library, so there's
no need for it to be linked into librustrt and available to all crates by
default.