Update Rayon dependency to 1.0

[killercup]

• [ ] Bump rayon's version number
• [ ] Figure out if this is a breaking change to quicli
  • Is there code out there that will no longer compile?

[CAD97]

Breaking changes

• Rayon now requires rustc 1.13 or greater.
• IndexedParallelIterator::len and ParallelIterator::opt_len now operate on &self instead of &mut self.
• IndexedParallelIterator::collect_into is now collect_into_vec.
• IndexedParallelIterator::unzip_into is now unzip_into_vecs.
• Rayon no longer exports the deprecated Configuration and initialize from rayon-core.

rustc 1.13 or greater

If quicli already requires rustc >= 1.13, we're good here. That doesn't seem to be explicitly documented yet.

IndexedParallelIterator::{len, collect_into[_vec], unzip_into[_vec]} and ParallelIterator::opt_len

Both traits are in rayon::prelude and are thus publicly re-exported from quicli, making a breaking change there a breaking change to quicli.

Configuration and initialize

Were not exported in rayon::prelude, so doesn't effect the stability of quicli.

[killercup]

quicli re-exports proc-macros, so it requires 1.15+ (I've only tested it with the latest stable, though.)

Alright, it's a major version bump, then. I'm totally fine with that, we still have some version numbers left to use :)

[ariasuni]

As #73 was merged, you should probably close this issue, right?

[killercup]

Yep! Thanks :)