Unique::new now returns Option

[crawfxrd]

Ref: rust-lang/rust/pull/42959

Unique::new was changed to return an Option<Self>.
Unique::new_unchecked offers the same behavior as the old new.

I am up to Printing Characters. print_something does not compile using rustc 1.21.0-nightly (52a330969 2017-07-27) due to this change.

[phil-opp]

Thanks for reporting! Yeah, we should change it to new_unchecked or maybe new().unwrap() to avoid the unsafety.

I'm on vacation until end of August, so I won't be able to fix this until then. However, I should be able to merge a pull request if someone wants to send one.