Implement `#[quickcheck]` syntax extension
General fixups & updates for the syntax extension.

- updates syntax & compiler interactions (no @, plugin API has changed, etc.)
- integrates the macro crate into the build system
- adds the example to the example folder, with the appropriate special
  handling
Override fill_bytes and next_u64 for StdGen.

Some RNGs override these for efficiency, so we might as well defer
directly to them.
Use a macro for the signed/unsigned integer Arbitrary impls.
Merge pull request #12 from huonw/random-patches

Random patches (incl. #[quickcheck] syntax extension)