Cargo ify (feedback request)

Request for comments. This is the cargo-ified version of rosetta-code.
The main reason I did it is because of the num library, that we use in various tasks for BigInts, being now a separate dependency (and slated from removal from the standard library). Cargo allows easily to specify it as a dependency. There are other tasks too that I would have wanted to do, but that require external graphical libraries (that I didn't know how to include with the current build system.

There are a few cons too:

  1 - we lose the pretty formatting of compilation and tests in the build output
  2 -  // not_tested does not do anything any longer. A #![allow(dead_code)] is now required for tasks that have no tests (or cargo test will warn about an unreachable "main()" )
  3 - the lints included in the build script (long lines and whitespace at the end of the line) are not run any longer