Warn for non-scalar const values

[jdm]

servo/servo#8086 (comment)

This seems like an easy thing to mess up.

[Manishearth]

Perhaps for sufficiently large arrays, at least (if not for all arrays)

[Manishearth]

To do this, you'll need to add a new lint (Somewhere in https://github.com/Manishearth/rust-clippy/blob/master/src/misc.rs), with a check_item, that checks if the type (via this) is a TyArray with large size (http://manishearth.github.io/rust-internals-docs/rustc/middle/ty/enum.TypeVariants.html).

[llogiq]

We have something similar in src/loops.rs, line 320.