remove use std::ascii::AsciiExt for Rust 1.23

[petertseng]

remove use std::ascii::AsciiExt for rust 1.23

In rust 1.23, the AsciiExt methods are implemented directly on the
concerned types, so attempting to import AsciiExt results in an
unused import.

Rust language PR:
rust-lang/rust#44042

PSA:
https://users.rust-lang.org/t/psa-dealing-with-warning-unused-import-std-ascii-asciiext-in-today-s-nightly/13726

This started warning as an unused import on nightly sometime between
2017-11-02 and 2017-11-09:

https://travis-ci.org/exercism/rust/builds/296391447
https://travis-ci.org/exercism/rust/builds/299770403

This started warning on beta exactly when 1.23.0 became Beta (it was
accepted on 1.22.0):

https://travis-ci.org/exercism/rust/builds/303139434
https://travis-ci.org/exercism/rust/builds/306431846

As 1.23 is now stable and our repo disallows warnings on stable, we will
need this.

[petertseng]

Well, I guess I kind of saw it coming. It doesn't compile on 1.22 but apparently will on 1.23. So when 1.23 becomes stable, since we disallow warnings on stable, our build will break until merging this PR.

Okay, well, we can't merge it until that time, so we shouldn't leave it open.

In the meantime, let's hunt for the change that made this happen.

[petertseng]

It's https://users.rust-lang.org/t/psa-dealing-with-warning-unused-import-std-ascii-asciiext-in-today-s-nightly/13726.

It's still the case that you must import a trait to use its methods (I was worried for a moment there), just that there are new inherent methods.

[petertseng]

It's time.

[coriolinus]

Agreed. These changes look good to me.

[petertseng]

I rewrote the commit message but made no other changes.