Implement is_odd and is_even instead of proxying to data

Trying to only make changes to rust-gmp when necessary.
Make the Makefile more generic

Should now work pretty well now for any library project set up like this:

  {project name}/
    Makefile
    src/
      {project name}/
        lib.rs
        ...
      examples/
        ...
    build/
      lib{project name}-{hash}-{version}.rlib
      examples/
        ...