... and other minor refactoring.
`bench_recursion` will be the default bin run by `cargo run`; the otheres can be selected with the `--bin` flag.
We could probably delete some of the other binaries later. E.g. `field_search` might not be useful any more. `bench_fft` should maybe be converted to a benchmark (although there are some pros and cons, e.g. the bench framework has a minimum number of runs, and isn't helpful in testing multi-core performance).
