circular buffer: remove allow(unused_must_use) (#490)
circular buffer: remove allow(unused_must_use) (#490)

* circular buffer: remove allow(unused_must_use)

This PR removes the compiler directive. Instead, all otherwise unused
`Result`s are now `unwrap()`'d.

Closes #475.

* Replace `unwrap()` with `assert!(....is_ok())`

This is more explicit, and requires less justification.

petertseng/exercism-rust,petertseng/exercism-rust,exercism/xrust,exercism/xrust,petertseng/exercism-rust