Optimize deps: Drop error_chain and move assert_approximate_eq to dev-deps

[w-flo]

• 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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Well, first of all, thanks again for the gpx crate! I've noticed the latest version parses gpx files that use some extensions without throwing an error, and I can access the time without going through a parse/format cycle, so it's even better now compared to last time I participated in this repo.

I'm not sure how valuable a changelog entry for this would be for users, and English is not my native language. So I haven't added one. Let me know if I should, or feel free to add it yourself if you want :-)

I've noticed these deps do not appear to be needed. The error_chain dependency seems especially "wasteful" because it brings in a bunch of transitive dependencies. Commit messages:

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Move `assert_approx_eq` crate to dev-dependencies

The crate is used in two test cases and appears not to be needed
otherwise.

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Remove error_chain remnants

The code no longer depends on error_chain since #58, but the dependency
was still listed in Cargo.toml. There still was one commented-out "use"
directive.

Drops the number of build steps from 69 to 48, and helps with
compilation time a bit.

[michaelkirk]

Thanks!

bors r+

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