Fix rustc 1.26 warnings
Fix rustc 1.26 warnings

This rustc release has deprecated the AsciiExt trait.
It turns out that the diamond exercise used that trait, and
therefore no longer compiles under DENYWARNINGS=1.

This removes its use of the deprecated trait.

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