Check non-trivial use std:: statements

[hobofan]

Anything beyond a module use statement (use std::ops;) or a single item use statement (use std::ops::Add;) is not handled.

What still needs to be implemented:

• glob use statements: use std::ops::*
• multi-use statements: use std::ops::{Add, Mul};
• a mixture of those use std::ops::{self, Add, Mul, fictional_mod::Trait, fictional_mod2::*};

Multi-use statements should be expanded to a list of equivalent single-use statements, on which we can then do further checks.

For glob use statements, we can't expand those (as we don't have enough information about public items in other modules. We can probably still handle those similarly to how we handle module use statements. This makes some assumptions e.g. the items between std::ops::* and core::ops::* stay the same when making replacement suggestions.