Extend arithmetic operator traits over refs

[Chris00]

This is convenient and std does it for standard types.

[unageek]

@Chris00 Could you investigate the Clippy errors?

[coveralls]

Pull Request Test Coverage Report for Build 1637422554

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 83.667%

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Totals
Change from base Build 1591111864:	0.0%
Covered Lines:	2004
Relevant Lines:	2044

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💛 - Coveralls

[Chris00]

@Chris00 Could you investigate the Clippy errors?

Done. They were about the #[must_use] attribute when returning Self.

[unageek]

LGTM! Thank you so much!