Rust binding to HdrHistogram_c library.

This is a thin binding to the HdrHistogramc library (https://github.com/HdrHistogram/HdrHistogramc). Aside from the normal Rust safety features, the main embellishment is an implementation of the Iterator trait for the various ways to iterate the histogram.

It also uses u64 instead of a signed type for values, as the library does not allow values to be less than 1. However it also means that any value greater than 2^63 will be treated as negative and rejected.

I've re-implemented the test suite in Rust (cargo test) to exercise the API, and it all passes.

TODO: * Finish basic API * Complete iterator items * Histogram logging/serialization * anything missing