Check for use std:: statements in crate itself

[hobofan]

Just adding #![no_std] to a crate is not enough. Especially in the main crate, we should also scan for use std:: statements and report those. Optimally the help message should also suggest switching from std:: to core:: if possible.

See also this Reddit thread and this commit, of strength_reduce adding no_std support.

[hobofan]

One special case would be going from std::f32 to core::f32, and similar switches of number types. As encountered in vislyhq/stretch#11, this requires using libm which we then should suggest, as core is currently lacking support for maths.

[hobofan]

#12 added a primitive version of this check

Stuff that's still to do there:

• Pretty print location of all offenses See feat(print): print location of use std:: statements #14
• More helpful error message with suggestions on how to fix the offenses. See feat(help): add suggestions for replacing use std:: with `use core:… #15. Implements use std:: -> use core:: suggestions where possible. Further suggestions should get their own issues.
• Check all files (not just entry file) for main crate. See cf0603b.