Crate attribute to use a custom prelude when using no_std

[thepowersgang]

For example: #![prelude="core::prelude"] to use the core's prelude, or #[prelude="my_prelude::v1"] to use a custom prelude.

This would probably load in the same way as the usual prelude (a non-exporting glob import into all sub-modules)

[gkoz]

Seems like #890 covers this use case.

[thepowersgang]

It does, closing as that RFC does cover this usecase.