Add Hough transform

Implemented the Hough Transform sample, adding the input image from the Rosetta website. Rather than use PNG and introduce another dependency, I converted it to PGM and wrote simple load/save support in Rust.  Compares exactly with the output of the Go version (on which it was based).
Added this and also fixed the `dead_code` warning on the test build. Running a Cargo build, test and bench are all clean.