Should fix unit tests by default as well

[alexcrichton]

Currently I think the suggestion is to run cargo fix but this currently means that unit tests, benchmarks, etc, will not be fixed. This can be a poor experience where you fix your way into the 2018 edition only to hit a roadblock when you execute cargo test.

I see one of two possibilities here:

• First up we could change cargo fix to automatically fix all Cargo targets by default. I like this in that it's sort of what I think we want, but I also don't like this in that it's still not guaranteed to fix everything. We'd need to encourage users to also do things like cargo fix -- --all-targets --all-features (in today's sytnax)
• We could alternatively add documentation to the edition guide suggesting that before enabling edition = '2018' you should attempt to fix all targets. This would involve both the --all-targets flag (perhaps) as well as things like --feature foo for the specific local crate.

I'm personally leaning towards the latter, but I'm curious what others think!

cc @killercup

[alexcrichton]

Hm bah but currently you also have to write #![feature(rust_2018_preview)] everywhere which is a bummer...

[Mark-Simulacrum]

Closing as duplicate of #19.