Seq linter: 1:length(...) and similar expressions

[gaborcsardi]

It finds 1:length(...), 1:nrow(...), 1:ncol(...), 1:NROW(...) and 1:NCOL(...) expressions, everywhere.

Maybe we should restrict it into for loop conditions, I am not sure.

It depends on the XML data, thus the #154 PR.

I'll add a bullet to the NEWS.

[jimhester]

Thanks Gabor, this is awesome, now I need to go back and rewrite some of the old linters using XPath. I imagine it will make the code cleaner and probably faster as well...

[gaborcsardi]

Thanks for merging! I'll add more linters that use this.