Use extend_from_slice instead of iterator extend

[fhartwig]

Extending a Vec using extend_from_slice is much faster than extending from an iterator. This reduces the time needed for quoting a string by 40-65% (40% on 50-char strings, 65% on 1000-char strings) if the char contains no escape characters. Also, I think it looks nicer 😃

[fhartwig]

Incidentally, replacing the s.iter().position(|&v| v == self.quote) in the same method by memchr(self.quote, s) reduces the runtime by an order of magnitude. I wasn't sure if you consider quoting performance to be worth the extra dependency, though. I'd be happy to make that change as well if you think it's worth it.

[BurntSushi]

@fhartwig This is good work, and I'd probably accept the PR, but this is undoubtedly a breaking change since this probably increases the minimum Rust version required to compile csv. I guess I'm not opposed to merging this and doing a semver incompatible bump, since extend_from_slice has been around since 1.6.

Incidentally, replacing the s.iter().position(|&v| v == self.quote) in the same method by memchr(self.quote, s) reduces the runtime by an order of magnitude. I wasn't sure if you consider quoting performance to be worth the extra dependency, though. I'd be happy to make that change as well if you think it's worth it.

This sounds like a good idea and I'd definitely be happy to add memchr as a dependency. If it's convenient for you, adding it into this PR is fine.

[fhartwig]

I've pushed another commit for the memchr change. I only made this PR because I thought it was a no-downsides performance improvement, but I hadn't considered compatibility with older compilers, so feel free to close this if you prefer.

[BurntSushi]

Awesome, thank you!

Backwards compatibility is important, but it's been a while since we had a version bump, so I'm fine with doing that.