Add lint long_variable_names

[JurrianFahner]

The lint is configurable by using max-variable-name-length.

Partly fixes #644 this PR only concerns the variable length

changelog: [long_variable_names]: enable variable length check in clippy

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[JurrianFahner]

r? @llogiq thanks for your clippy workshop today! Looking forward for your feedback.

[JurrianFahner]

I doubt whether the default value is well chosen. 30 seems ok for most of the use cases. But for the clippy source it was needed to create some different configurations too.

[llogiq]

Yeah, so I'd like to make sure that we have some benefit if we actually have that lint warn. I figure we either make it a pedantic lint or we set a far higher default threshold. When in doubt, you can run lintcheck.

Otherwise this looks good to merge.

[llogiq]

I think if we make it warn by default, we better pick a really large threshold. Adding warnings to many folks' CI jobs should have some serious benefit to make up for the hassle. So I'd either make it pedantic (which is allow by default) or set the default absurdly high, e.g. 100, so people who think this is a good idea can introduce tighter bounds to their projects.