Write speed to GPX 1.0 files

[tpikonen]

• I agree to follow the project's code of conduct.
• I added an entry to CHANGELOG.md if knowledge of this change could be valuable to users.

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Does what it says in the tin.

[lnicola]

Maybe we should thread only the version, instead of the whole &Gpx?

[tpikonen]

I'm pretty new to Rust, but I would be surprised if function arguments are copied into the stack, especially when the value is a reference.

So, this should not be a performance issue, but maybe there are other reasons to not pass the whole type?

[lnicola]

No, I'm not worried about performance. Passing the version might be a couple % faster if you ignore the I/O, but that's not the issue.

I'm thinking that we should only pass the necessary information instead of the whole thing, so the code doesn't end up looking where it shouldn't.

[tpikonen]

Here's a version which passes only gpx.version.

[lnicola]

Looks good to me with a single nit. I'll wait a couple of days before merging in case another maintainer wants to take a look.

[lnicola]

r? @frewsxcv