In-progress rewrite of the random number library in Rust's stdlib.

See https://github.com/mozilla/rust/wiki/Lib-rand for (some) info.

Currently the architecture is a bit strange. Requires a recent
incoming build.

# Building

To build the shared library

   make

To run benchmarks,

   make bench

   make lib-bench
   make ext-bench


The first runs all benchmarks. The second runs the microbenchmarks in
the code itself (i.e. the tests anontated with #[bench]), and the last
runs the tests in ./bench/, specifically it compares the Rust
implementations with other implementations (e.g. the upstream C ones).