Rust support hits linux-next

Followers of the linux-next integration tree may have noticed a significantaddition: initial support for writing device drivers in the Rust language.There is some documentation in Documentation/rust,while the code itself is in the rusttop-level directory. Appearance in linux-next generally implies readinessfor the upcoming merge window, but it is not clear if that is the casehere; this code has not seen a lot of wider review yet. It is, regardless,an important step toward the ability to write drivers in a safer language.