New for_loop_index_linter

[MichaelChirico]

Part of #884

This one is not violated often in our experience, but it's quite bad when it is.

x <- 1:10
for (x in x) x
x
# [1] 10

Seems like undesirable behavior.

[IndrajeetPatil]

LGTM.

The only nitpick I have is using `for()` instead of for in the docs and message. But don't feel strongly about it.

[MichaelChirico]

"for loop" is a much more general programming construct, for() doesn't look right to me. I'll keep it as is. Thanks!

[IndrajeetPatil]

Sure, but I find it easier to parse the following

This is true for for() loops.

over this

This is true for for loops.

[MichaelChirico]

Sure, I would disambiguate with "for for loops", but that's not used here anyway?