use copy_within instead of manual logic

[folkertdev]

Just a small readability win.

https://godbolt.org/z/9s3zMebrE

• For memmove16, the assembly is basically equivalent for the happy path
• For memcpy_within_slice the standard path that calls memmove has fewer instructions

In practice this logic is apparently not exercised enough to show any measurable effect

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Btw I'm looking through your code base as a part of this rust project goal rust-lang/rust-project-goals#258. We've learned some things in zlib-rs that might be useful here too.

[CLAassistant]

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