Add memmem

[vickenty]

memmem is a non-standard extension, first added in GNU libc and later
ported to other systems. Support for it is non-uniform, thus it was only
added to platforms that seem to support it.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @gnzlbg (or someone else) soon.

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[JohnTitor]

Thanks!