When building with `no_std`, `alloc` imports don't also add `extern crate alloc`

[Lokathor]

When working on a #![no_std] crate and doing a code complete on Vec, selecting alloc::vec::Vec, the import is added to the top of the file, but an appropriate extern crate alloc; is not added anywhere, causing the project to immediately complain and fail to build.

It would be better if RA could detect this and add the extern crate at the crate root.