refactor(lib): Reduce API surface and unsafe code

- Add dependency on parallel.rs
- Use the safe parallel abstractions defined in parallel.rs
- Move distribution::Distribution to the root of the crate
- Remove `bootstrap{,2}_many` methods, use `bootstrap{,2}` instead and then use
  the `split{2,4}` method.

[breaking-change]
Merge pull request #10 from japaric/refactor

refactor(lib): Reduce API surface and unsafe code