Allow deprecated to remove warnings for rustc 1.26 onwards

[guangie88]

Currently using arg_enum macro on nightly will trigger the following warning:

use of deprecated item 'std::ascii::AsciiExt': use inherent methods instead

This is due to https://github.com/rust-lang/rust/blob/master/src/libstd/ascii.rs#L55. Projects will fail to compile on nightly if #![deny(warnings)] is set.

Resolve by appending #[allow(deprecated)] to affected parts to prevent warnings from bubbling up to the projects using the library.

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This change is 

[mention-bot]

@guangie88, thanks for your PR! By analyzing the history of the files in this pull request, we identified @kbknapp to be a potential reviewer.

[kbknapp]

Thanks 👍