Clean up some unsafe code introduced by #202

[bugadani]

Although I haven't checked these exact functions, a simple clone seems to be enough to elide the bounds checks: https://godbolt.org/z/a1jb4h (access taken directly from the issue).

[rust-highfive]

r? @adamgreig

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[bugadani]

Actually, deriving Copy along with clone messes things up: https://godbolt.org/z/zhYfEs